Finding the tenth derivative
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I came across this Question where i have to find $$f^{(10)}$$ for the following function at $x = 0$
$$f(x) = e^xsin x$$
I tried differentiating a few times to get a pattern but didn’t get one, can someone provide the solution.
calculus
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add a comment |
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I came across this Question where i have to find $$f^{(10)}$$ for the following function at $x = 0$
$$f(x) = e^xsin x$$
I tried differentiating a few times to get a pattern but didn’t get one, can someone provide the solution.
calculus
$endgroup$
add a comment |
$begingroup$
I came across this Question where i have to find $$f^{(10)}$$ for the following function at $x = 0$
$$f(x) = e^xsin x$$
I tried differentiating a few times to get a pattern but didn’t get one, can someone provide the solution.
calculus
$endgroup$
I came across this Question where i have to find $$f^{(10)}$$ for the following function at $x = 0$
$$f(x) = e^xsin x$$
I tried differentiating a few times to get a pattern but didn’t get one, can someone provide the solution.
calculus
calculus
edited 9 hours ago
Shalop
9,21411030
9,21411030
asked 21 hours ago
user601297user601297
3509
3509
add a comment |
add a comment |
9 Answers
9
active
oldest
votes
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Hint:
$$f(x)=e^xsin x$$
$$f'(x)=e^x(sin x +cos x)$$
$$f''(x)=e^x(sin x+cos x)+e^x(cos x -sin x)=2e^x(cos x)$$
$$f'''(x)=e^x(2cos x)-e^x(2sin x)=2e^x(cos x-sin x)$$
$$f^{IV}(x)=2e^x(cos x-sin x)-2e^x(cos x+sin x)=-4e^x(sin x)=-4f(x)$$
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10
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First time to see roman numerals to denote derivatives. Nice!
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– Taladris
19 hours ago
13
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Lower case too, awful notation if you ask me.
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– marshal craft
13 hours ago
4
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I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
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– ilkkachu
9 hours ago
14
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There will be some confusion when you get to the tenth derivative......
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– Spencer
7 hours ago
5
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@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
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– BPP
4 hours ago
|
show 2 more comments
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Hint:
As $;mathrm e^xsin x=operatorname{Im}bigl(mathrm e^{(1+i)x}bigr)$, you have to find first the real and imaginary parts of $(1+i)^{10}$.
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12
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This is by far the most efficient solution.
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– Yves Daoust
21 hours ago
8
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Knowing $(1+i)^2=2i$ helps too
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– Henry
12 hours ago
1
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This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
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– Peter LeFanu Lumsdaine
12 hours ago
add a comment |
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Using power series: it is well-known that $e^x=sum_{k=0}^infty frac{x^k}{k!}$ and $sin(x)=sum_{k=0}^infty (-1)^kfrac{x^{2k+1}}{(2k+1)!}$ for any real number $x$, so
$$ e^xsin(x)=(1+x+frac{x^2}{2!}+dots+frac{x^{10}}{10!}+dots)(x-frac{x^3}{3!}+frac{x^5}{5!}-frac{x^7}{7!}+frac{x^9}{9!}+dots) $$
By expanding, the coefficient of $x^{10}$ is $frac{1}{9!1!}-frac{1}{7!3!}+frac{1}{5!5!}-frac{1}{7!3!}+frac{1}{9!1!}$
But this coefficient is also $frac{f^{(10)}(0)}{10!}$, so
$$ f^{(10)}(0)=frac{10!}{9!1!}-frac{10!}{7!3!}+frac{10!}{5!5!}-frac{10!}{7!3!}+frac{10!}{9!1!} = 10 -120 + 252 - 120 +10 = 32$$
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2
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Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
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– Count Iblis
17 hours ago
add a comment |
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Use Leibniz' Rule for higher derivatives of a product:
$$frac{d^n}{dx^n}(uv)
=frac{d^nu}{dx^n}v+binom n1frac{d^{n-1}u}{dx^{n-1}}frac{dv}{dx}
+binom n2frac{d^{n-2}u}{dx^{n-2}}frac{d^2v}{dx^2}+cdots+ufrac{d^nv}{dx^n} .$$
In your case take $u=e^x$ and $v=sin x$. Since you are going to substitute $x=0$ after differentiating, all the $e^x$ terms will be $1$, all the $sin x$ terms will be $0$ and all the cos $x$ terms will be $1$ (though some of them will pick up a negative sign when you differentiate). So the answer is
$$eqalign{0+binom{10}11&{}+binom{10}20+binom{10}3(-1)+binom{10}40+binom{10}51cr
&qquad{}+binom{10}60+binom{10}7(-1)+binom{10}80+binom{10}91+binom{10}{10}0cr
&=10-120+252-120+10cr
&=32 .cr}$$
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add a comment |
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$$(e^x(acos x+bsin x))'=e^x(acos x+bsin x)+e^x(bcos x-asin x)=e^x((a+b)cos x+(b-a)sin x).$$
So
$$(0,1)to(1,1)to(2,0)to(2,-2)to(0,-4)to(-4,-4)to(-8,0)to(-8,8)to(0,16)to(16,16)to(32,0).$$
If you divide by increasing powers of $2$, in pairs, the pattern emerges, with period $8$:
$$(0,1)to(1,1)to(1,0)to(1,-1)to(0,-1)to(-1,-1)to(-1,0)to(-1,1)to(0,1)to(1,1)to(1,0).$$
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add a comment |
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One trick here is to use $e^{ix}=cos x+isin x$ and define $g(x)=e^xcos x$ then $f(x)$ and $g(x)$ are both real functions.
Let $h(x)=g(x)+if(x)=e^{(1+i)x}$ then the tenth derivative of $h(x)$ is $(1+i)^{10}h(x)$ and the tenth derivative of $f(x)$ is the imaginary part of this.
Because you only want the value at $x=0$ you can evaluate there, with $h(0)=g(0)+if(0)$
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Use the formula $(fg)^{(n)}= sumlimits_{k=0}^{n} binom {n} {k} (f)^{(k)}(g)^{(n-k)}$.
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add a comment |
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I get $f^{10}(x)=32e^xcos x$.
Here's what I did:
begin{align}f'(x)&=e^x(sin x+cos x)\
implies f''(x)&=2e^xcos x\
implies f^3(x)&=2e^x(cos x-sin x)\
implies f^4(x)&=2e^x(-2sin x)=-4f(x)\
implies f^5(x)&=-4f'(x)\
implies f^8(x)&=-4f^4(x)=16f(x)\
implies f^{10}(x)&=16f''(x)end{align}
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Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
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– AlexanderJ93
21 hours ago
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Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
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– Rhys Hughes
21 hours ago
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@RhysHughes &Alexander J93 thanks
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– Chris Custer
21 hours ago
add a comment |
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Regarding the question about looking for a pattern:
Repeated application of the product rule gives
$$ f(x) = e^x sin x$$
$$ f'(x) = e^x sin x + e^x cos x$$
$$ f''(x) = e^x sin x + e^x cos x + e^x cos x - e^x sin x = 2 e^x cos x$$
Is a pattern emerging?
$$ f^{(3)}(x) = 2 e^x cos x -2 e^x sin x$$
$$ f^{(4)}(x) = 2 e^x cos x -2 e^x sin x - 2 e^x sin x - 2 e^x cos x = -4 e^x sin x$$
Yes. We can then conclude that
$ f^{(6)}(x) = -8 e^x cos x$, $f^{(8)}(x) = 16 e^x sin x$, and $f^{(10)}(x) = 32 e^x cos x$
such that $f^{(10)}(0) = 32 $
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9 Answers
9
active
oldest
votes
9 Answers
9
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint:
$$f(x)=e^xsin x$$
$$f'(x)=e^x(sin x +cos x)$$
$$f''(x)=e^x(sin x+cos x)+e^x(cos x -sin x)=2e^x(cos x)$$
$$f'''(x)=e^x(2cos x)-e^x(2sin x)=2e^x(cos x-sin x)$$
$$f^{IV}(x)=2e^x(cos x-sin x)-2e^x(cos x+sin x)=-4e^x(sin x)=-4f(x)$$
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10
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First time to see roman numerals to denote derivatives. Nice!
$endgroup$
– Taladris
19 hours ago
13
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Lower case too, awful notation if you ask me.
$endgroup$
– marshal craft
13 hours ago
4
$begingroup$
I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
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– ilkkachu
9 hours ago
14
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There will be some confusion when you get to the tenth derivative......
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– Spencer
7 hours ago
5
$begingroup$
@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
$endgroup$
– BPP
4 hours ago
|
show 2 more comments
$begingroup$
Hint:
$$f(x)=e^xsin x$$
$$f'(x)=e^x(sin x +cos x)$$
$$f''(x)=e^x(sin x+cos x)+e^x(cos x -sin x)=2e^x(cos x)$$
$$f'''(x)=e^x(2cos x)-e^x(2sin x)=2e^x(cos x-sin x)$$
$$f^{IV}(x)=2e^x(cos x-sin x)-2e^x(cos x+sin x)=-4e^x(sin x)=-4f(x)$$
$endgroup$
10
$begingroup$
First time to see roman numerals to denote derivatives. Nice!
$endgroup$
– Taladris
19 hours ago
13
$begingroup$
Lower case too, awful notation if you ask me.
$endgroup$
– marshal craft
13 hours ago
4
$begingroup$
I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
$endgroup$
– ilkkachu
9 hours ago
14
$begingroup$
There will be some confusion when you get to the tenth derivative......
$endgroup$
– Spencer
7 hours ago
5
$begingroup$
@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
$endgroup$
– BPP
4 hours ago
|
show 2 more comments
$begingroup$
Hint:
$$f(x)=e^xsin x$$
$$f'(x)=e^x(sin x +cos x)$$
$$f''(x)=e^x(sin x+cos x)+e^x(cos x -sin x)=2e^x(cos x)$$
$$f'''(x)=e^x(2cos x)-e^x(2sin x)=2e^x(cos x-sin x)$$
$$f^{IV}(x)=2e^x(cos x-sin x)-2e^x(cos x+sin x)=-4e^x(sin x)=-4f(x)$$
$endgroup$
Hint:
$$f(x)=e^xsin x$$
$$f'(x)=e^x(sin x +cos x)$$
$$f''(x)=e^x(sin x+cos x)+e^x(cos x -sin x)=2e^x(cos x)$$
$$f'''(x)=e^x(2cos x)-e^x(2sin x)=2e^x(cos x-sin x)$$
$$f^{IV}(x)=2e^x(cos x-sin x)-2e^x(cos x+sin x)=-4e^x(sin x)=-4f(x)$$
edited 5 hours ago
answered 21 hours ago
Rhys HughesRhys Hughes
5,4491528
5,4491528
10
$begingroup$
First time to see roman numerals to denote derivatives. Nice!
$endgroup$
– Taladris
19 hours ago
13
$begingroup$
Lower case too, awful notation if you ask me.
$endgroup$
– marshal craft
13 hours ago
4
$begingroup$
I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
$endgroup$
– ilkkachu
9 hours ago
14
$begingroup$
There will be some confusion when you get to the tenth derivative......
$endgroup$
– Spencer
7 hours ago
5
$begingroup$
@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
$endgroup$
– BPP
4 hours ago
|
show 2 more comments
10
$begingroup$
First time to see roman numerals to denote derivatives. Nice!
$endgroup$
– Taladris
19 hours ago
13
$begingroup$
Lower case too, awful notation if you ask me.
$endgroup$
– marshal craft
13 hours ago
4
$begingroup$
I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
$endgroup$
– ilkkachu
9 hours ago
14
$begingroup$
There will be some confusion when you get to the tenth derivative......
$endgroup$
– Spencer
7 hours ago
5
$begingroup$
@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
$endgroup$
– BPP
4 hours ago
10
10
$begingroup$
First time to see roman numerals to denote derivatives. Nice!
$endgroup$
– Taladris
19 hours ago
$begingroup$
First time to see roman numerals to denote derivatives. Nice!
$endgroup$
– Taladris
19 hours ago
13
13
$begingroup$
Lower case too, awful notation if you ask me.
$endgroup$
– marshal craft
13 hours ago
$begingroup$
Lower case too, awful notation if you ask me.
$endgroup$
– marshal craft
13 hours ago
4
4
$begingroup$
I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
$endgroup$
– ilkkachu
9 hours ago
$begingroup$
I have no problem with lowercase roman numerals in general, but here it looks like taking the power to the imaginary unit (times $v$)...
$endgroup$
– ilkkachu
9 hours ago
14
14
$begingroup$
There will be some confusion when you get to the tenth derivative......
$endgroup$
– Spencer
7 hours ago
$begingroup$
There will be some confusion when you get to the tenth derivative......
$endgroup$
– Spencer
7 hours ago
5
5
$begingroup$
@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
$endgroup$
– BPP
4 hours ago
$begingroup$
@Shufflepants en.wikipedia.org/wiki/Fractional_calculus
$endgroup$
– BPP
4 hours ago
|
show 2 more comments
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Hint:
As $;mathrm e^xsin x=operatorname{Im}bigl(mathrm e^{(1+i)x}bigr)$, you have to find first the real and imaginary parts of $(1+i)^{10}$.
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12
$begingroup$
This is by far the most efficient solution.
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– Yves Daoust
21 hours ago
8
$begingroup$
Knowing $(1+i)^2=2i$ helps too
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– Henry
12 hours ago
1
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This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
$endgroup$
– Peter LeFanu Lumsdaine
12 hours ago
add a comment |
$begingroup$
Hint:
As $;mathrm e^xsin x=operatorname{Im}bigl(mathrm e^{(1+i)x}bigr)$, you have to find first the real and imaginary parts of $(1+i)^{10}$.
$endgroup$
12
$begingroup$
This is by far the most efficient solution.
$endgroup$
– Yves Daoust
21 hours ago
8
$begingroup$
Knowing $(1+i)^2=2i$ helps too
$endgroup$
– Henry
12 hours ago
1
$begingroup$
This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
$endgroup$
– Peter LeFanu Lumsdaine
12 hours ago
add a comment |
$begingroup$
Hint:
As $;mathrm e^xsin x=operatorname{Im}bigl(mathrm e^{(1+i)x}bigr)$, you have to find first the real and imaginary parts of $(1+i)^{10}$.
$endgroup$
Hint:
As $;mathrm e^xsin x=operatorname{Im}bigl(mathrm e^{(1+i)x}bigr)$, you have to find first the real and imaginary parts of $(1+i)^{10}$.
answered 21 hours ago
BernardBernard
119k740113
119k740113
12
$begingroup$
This is by far the most efficient solution.
$endgroup$
– Yves Daoust
21 hours ago
8
$begingroup$
Knowing $(1+i)^2=2i$ helps too
$endgroup$
– Henry
12 hours ago
1
$begingroup$
This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
$endgroup$
– Peter LeFanu Lumsdaine
12 hours ago
add a comment |
12
$begingroup$
This is by far the most efficient solution.
$endgroup$
– Yves Daoust
21 hours ago
8
$begingroup$
Knowing $(1+i)^2=2i$ helps too
$endgroup$
– Henry
12 hours ago
1
$begingroup$
This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
$endgroup$
– Peter LeFanu Lumsdaine
12 hours ago
12
12
$begingroup$
This is by far the most efficient solution.
$endgroup$
– Yves Daoust
21 hours ago
$begingroup$
This is by far the most efficient solution.
$endgroup$
– Yves Daoust
21 hours ago
8
8
$begingroup$
Knowing $(1+i)^2=2i$ helps too
$endgroup$
– Henry
12 hours ago
$begingroup$
Knowing $(1+i)^2=2i$ helps too
$endgroup$
– Henry
12 hours ago
1
1
$begingroup$
This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
$endgroup$
– Peter LeFanu Lumsdaine
12 hours ago
$begingroup$
This is an excellent hint, for students with a little knowledge of complex numbers. It also can be followed by a second hint: don’t use binomial expansion to find these!
$endgroup$
– Peter LeFanu Lumsdaine
12 hours ago
add a comment |
$begingroup$
Using power series: it is well-known that $e^x=sum_{k=0}^infty frac{x^k}{k!}$ and $sin(x)=sum_{k=0}^infty (-1)^kfrac{x^{2k+1}}{(2k+1)!}$ for any real number $x$, so
$$ e^xsin(x)=(1+x+frac{x^2}{2!}+dots+frac{x^{10}}{10!}+dots)(x-frac{x^3}{3!}+frac{x^5}{5!}-frac{x^7}{7!}+frac{x^9}{9!}+dots) $$
By expanding, the coefficient of $x^{10}$ is $frac{1}{9!1!}-frac{1}{7!3!}+frac{1}{5!5!}-frac{1}{7!3!}+frac{1}{9!1!}$
But this coefficient is also $frac{f^{(10)}(0)}{10!}$, so
$$ f^{(10)}(0)=frac{10!}{9!1!}-frac{10!}{7!3!}+frac{10!}{5!5!}-frac{10!}{7!3!}+frac{10!}{9!1!} = 10 -120 + 252 - 120 +10 = 32$$
$endgroup$
2
$begingroup$
Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
$endgroup$
– Count Iblis
17 hours ago
add a comment |
$begingroup$
Using power series: it is well-known that $e^x=sum_{k=0}^infty frac{x^k}{k!}$ and $sin(x)=sum_{k=0}^infty (-1)^kfrac{x^{2k+1}}{(2k+1)!}$ for any real number $x$, so
$$ e^xsin(x)=(1+x+frac{x^2}{2!}+dots+frac{x^{10}}{10!}+dots)(x-frac{x^3}{3!}+frac{x^5}{5!}-frac{x^7}{7!}+frac{x^9}{9!}+dots) $$
By expanding, the coefficient of $x^{10}$ is $frac{1}{9!1!}-frac{1}{7!3!}+frac{1}{5!5!}-frac{1}{7!3!}+frac{1}{9!1!}$
But this coefficient is also $frac{f^{(10)}(0)}{10!}$, so
$$ f^{(10)}(0)=frac{10!}{9!1!}-frac{10!}{7!3!}+frac{10!}{5!5!}-frac{10!}{7!3!}+frac{10!}{9!1!} = 10 -120 + 252 - 120 +10 = 32$$
$endgroup$
2
$begingroup$
Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
$endgroup$
– Count Iblis
17 hours ago
add a comment |
$begingroup$
Using power series: it is well-known that $e^x=sum_{k=0}^infty frac{x^k}{k!}$ and $sin(x)=sum_{k=0}^infty (-1)^kfrac{x^{2k+1}}{(2k+1)!}$ for any real number $x$, so
$$ e^xsin(x)=(1+x+frac{x^2}{2!}+dots+frac{x^{10}}{10!}+dots)(x-frac{x^3}{3!}+frac{x^5}{5!}-frac{x^7}{7!}+frac{x^9}{9!}+dots) $$
By expanding, the coefficient of $x^{10}$ is $frac{1}{9!1!}-frac{1}{7!3!}+frac{1}{5!5!}-frac{1}{7!3!}+frac{1}{9!1!}$
But this coefficient is also $frac{f^{(10)}(0)}{10!}$, so
$$ f^{(10)}(0)=frac{10!}{9!1!}-frac{10!}{7!3!}+frac{10!}{5!5!}-frac{10!}{7!3!}+frac{10!}{9!1!} = 10 -120 + 252 - 120 +10 = 32$$
$endgroup$
Using power series: it is well-known that $e^x=sum_{k=0}^infty frac{x^k}{k!}$ and $sin(x)=sum_{k=0}^infty (-1)^kfrac{x^{2k+1}}{(2k+1)!}$ for any real number $x$, so
$$ e^xsin(x)=(1+x+frac{x^2}{2!}+dots+frac{x^{10}}{10!}+dots)(x-frac{x^3}{3!}+frac{x^5}{5!}-frac{x^7}{7!}+frac{x^9}{9!}+dots) $$
By expanding, the coefficient of $x^{10}$ is $frac{1}{9!1!}-frac{1}{7!3!}+frac{1}{5!5!}-frac{1}{7!3!}+frac{1}{9!1!}$
But this coefficient is also $frac{f^{(10)}(0)}{10!}$, so
$$ f^{(10)}(0)=frac{10!}{9!1!}-frac{10!}{7!3!}+frac{10!}{5!5!}-frac{10!}{7!3!}+frac{10!}{9!1!} = 10 -120 + 252 - 120 +10 = 32$$
answered 18 hours ago
TaladrisTaladris
4,84231933
4,84231933
2
$begingroup$
Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
$endgroup$
– Count Iblis
17 hours ago
add a comment |
2
$begingroup$
Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
$endgroup$
– Count Iblis
17 hours ago
2
2
$begingroup$
Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
$endgroup$
– Count Iblis
17 hours ago
$begingroup$
Take note of this solution, if the derivative at some point is needed, you don't need a symbolic expression for the $n$th derivative evaluated at some arbitrary point $x$. In some cases where a symbolic expression is not available, you can still find the $n$th derivative at some special point for arbitrary $n$. Also, there exists a general method based on Newton-Raphson division to find the $n$th derivative of an arbitrary function at some given point using some power of $log(n)$ multiplications. Some computer algebra systems use such methods to compute series expansions.
$endgroup$
– Count Iblis
17 hours ago
add a comment |
$begingroup$
Use Leibniz' Rule for higher derivatives of a product:
$$frac{d^n}{dx^n}(uv)
=frac{d^nu}{dx^n}v+binom n1frac{d^{n-1}u}{dx^{n-1}}frac{dv}{dx}
+binom n2frac{d^{n-2}u}{dx^{n-2}}frac{d^2v}{dx^2}+cdots+ufrac{d^nv}{dx^n} .$$
In your case take $u=e^x$ and $v=sin x$. Since you are going to substitute $x=0$ after differentiating, all the $e^x$ terms will be $1$, all the $sin x$ terms will be $0$ and all the cos $x$ terms will be $1$ (though some of them will pick up a negative sign when you differentiate). So the answer is
$$eqalign{0+binom{10}11&{}+binom{10}20+binom{10}3(-1)+binom{10}40+binom{10}51cr
&qquad{}+binom{10}60+binom{10}7(-1)+binom{10}80+binom{10}91+binom{10}{10}0cr
&=10-120+252-120+10cr
&=32 .cr}$$
$endgroup$
add a comment |
$begingroup$
Use Leibniz' Rule for higher derivatives of a product:
$$frac{d^n}{dx^n}(uv)
=frac{d^nu}{dx^n}v+binom n1frac{d^{n-1}u}{dx^{n-1}}frac{dv}{dx}
+binom n2frac{d^{n-2}u}{dx^{n-2}}frac{d^2v}{dx^2}+cdots+ufrac{d^nv}{dx^n} .$$
In your case take $u=e^x$ and $v=sin x$. Since you are going to substitute $x=0$ after differentiating, all the $e^x$ terms will be $1$, all the $sin x$ terms will be $0$ and all the cos $x$ terms will be $1$ (though some of them will pick up a negative sign when you differentiate). So the answer is
$$eqalign{0+binom{10}11&{}+binom{10}20+binom{10}3(-1)+binom{10}40+binom{10}51cr
&qquad{}+binom{10}60+binom{10}7(-1)+binom{10}80+binom{10}91+binom{10}{10}0cr
&=10-120+252-120+10cr
&=32 .cr}$$
$endgroup$
add a comment |
$begingroup$
Use Leibniz' Rule for higher derivatives of a product:
$$frac{d^n}{dx^n}(uv)
=frac{d^nu}{dx^n}v+binom n1frac{d^{n-1}u}{dx^{n-1}}frac{dv}{dx}
+binom n2frac{d^{n-2}u}{dx^{n-2}}frac{d^2v}{dx^2}+cdots+ufrac{d^nv}{dx^n} .$$
In your case take $u=e^x$ and $v=sin x$. Since you are going to substitute $x=0$ after differentiating, all the $e^x$ terms will be $1$, all the $sin x$ terms will be $0$ and all the cos $x$ terms will be $1$ (though some of them will pick up a negative sign when you differentiate). So the answer is
$$eqalign{0+binom{10}11&{}+binom{10}20+binom{10}3(-1)+binom{10}40+binom{10}51cr
&qquad{}+binom{10}60+binom{10}7(-1)+binom{10}80+binom{10}91+binom{10}{10}0cr
&=10-120+252-120+10cr
&=32 .cr}$$
$endgroup$
Use Leibniz' Rule for higher derivatives of a product:
$$frac{d^n}{dx^n}(uv)
=frac{d^nu}{dx^n}v+binom n1frac{d^{n-1}u}{dx^{n-1}}frac{dv}{dx}
+binom n2frac{d^{n-2}u}{dx^{n-2}}frac{d^2v}{dx^2}+cdots+ufrac{d^nv}{dx^n} .$$
In your case take $u=e^x$ and $v=sin x$. Since you are going to substitute $x=0$ after differentiating, all the $e^x$ terms will be $1$, all the $sin x$ terms will be $0$ and all the cos $x$ terms will be $1$ (though some of them will pick up a negative sign when you differentiate). So the answer is
$$eqalign{0+binom{10}11&{}+binom{10}20+binom{10}3(-1)+binom{10}40+binom{10}51cr
&qquad{}+binom{10}60+binom{10}7(-1)+binom{10}80+binom{10}91+binom{10}{10}0cr
&=10-120+252-120+10cr
&=32 .cr}$$
answered 21 hours ago
DavidDavid
68.1k664126
68.1k664126
add a comment |
add a comment |
$begingroup$
$$(e^x(acos x+bsin x))'=e^x(acos x+bsin x)+e^x(bcos x-asin x)=e^x((a+b)cos x+(b-a)sin x).$$
So
$$(0,1)to(1,1)to(2,0)to(2,-2)to(0,-4)to(-4,-4)to(-8,0)to(-8,8)to(0,16)to(16,16)to(32,0).$$
If you divide by increasing powers of $2$, in pairs, the pattern emerges, with period $8$:
$$(0,1)to(1,1)to(1,0)to(1,-1)to(0,-1)to(-1,-1)to(-1,0)to(-1,1)to(0,1)to(1,1)to(1,0).$$
$endgroup$
add a comment |
$begingroup$
$$(e^x(acos x+bsin x))'=e^x(acos x+bsin x)+e^x(bcos x-asin x)=e^x((a+b)cos x+(b-a)sin x).$$
So
$$(0,1)to(1,1)to(2,0)to(2,-2)to(0,-4)to(-4,-4)to(-8,0)to(-8,8)to(0,16)to(16,16)to(32,0).$$
If you divide by increasing powers of $2$, in pairs, the pattern emerges, with period $8$:
$$(0,1)to(1,1)to(1,0)to(1,-1)to(0,-1)to(-1,-1)to(-1,0)to(-1,1)to(0,1)to(1,1)to(1,0).$$
$endgroup$
add a comment |
$begingroup$
$$(e^x(acos x+bsin x))'=e^x(acos x+bsin x)+e^x(bcos x-asin x)=e^x((a+b)cos x+(b-a)sin x).$$
So
$$(0,1)to(1,1)to(2,0)to(2,-2)to(0,-4)to(-4,-4)to(-8,0)to(-8,8)to(0,16)to(16,16)to(32,0).$$
If you divide by increasing powers of $2$, in pairs, the pattern emerges, with period $8$:
$$(0,1)to(1,1)to(1,0)to(1,-1)to(0,-1)to(-1,-1)to(-1,0)to(-1,1)to(0,1)to(1,1)to(1,0).$$
$endgroup$
$$(e^x(acos x+bsin x))'=e^x(acos x+bsin x)+e^x(bcos x-asin x)=e^x((a+b)cos x+(b-a)sin x).$$
So
$$(0,1)to(1,1)to(2,0)to(2,-2)to(0,-4)to(-4,-4)to(-8,0)to(-8,8)to(0,16)to(16,16)to(32,0).$$
If you divide by increasing powers of $2$, in pairs, the pattern emerges, with period $8$:
$$(0,1)to(1,1)to(1,0)to(1,-1)to(0,-1)to(-1,-1)to(-1,0)to(-1,1)to(0,1)to(1,1)to(1,0).$$
edited 21 hours ago
answered 21 hours ago
Yves DaoustYves Daoust
125k671222
125k671222
add a comment |
add a comment |
$begingroup$
One trick here is to use $e^{ix}=cos x+isin x$ and define $g(x)=e^xcos x$ then $f(x)$ and $g(x)$ are both real functions.
Let $h(x)=g(x)+if(x)=e^{(1+i)x}$ then the tenth derivative of $h(x)$ is $(1+i)^{10}h(x)$ and the tenth derivative of $f(x)$ is the imaginary part of this.
Because you only want the value at $x=0$ you can evaluate there, with $h(0)=g(0)+if(0)$
$endgroup$
add a comment |
$begingroup$
One trick here is to use $e^{ix}=cos x+isin x$ and define $g(x)=e^xcos x$ then $f(x)$ and $g(x)$ are both real functions.
Let $h(x)=g(x)+if(x)=e^{(1+i)x}$ then the tenth derivative of $h(x)$ is $(1+i)^{10}h(x)$ and the tenth derivative of $f(x)$ is the imaginary part of this.
Because you only want the value at $x=0$ you can evaluate there, with $h(0)=g(0)+if(0)$
$endgroup$
add a comment |
$begingroup$
One trick here is to use $e^{ix}=cos x+isin x$ and define $g(x)=e^xcos x$ then $f(x)$ and $g(x)$ are both real functions.
Let $h(x)=g(x)+if(x)=e^{(1+i)x}$ then the tenth derivative of $h(x)$ is $(1+i)^{10}h(x)$ and the tenth derivative of $f(x)$ is the imaginary part of this.
Because you only want the value at $x=0$ you can evaluate there, with $h(0)=g(0)+if(0)$
$endgroup$
One trick here is to use $e^{ix}=cos x+isin x$ and define $g(x)=e^xcos x$ then $f(x)$ and $g(x)$ are both real functions.
Let $h(x)=g(x)+if(x)=e^{(1+i)x}$ then the tenth derivative of $h(x)$ is $(1+i)^{10}h(x)$ and the tenth derivative of $f(x)$ is the imaginary part of this.
Because you only want the value at $x=0$ you can evaluate there, with $h(0)=g(0)+if(0)$
answered 21 hours ago
Mark BennetMark Bennet
80.8k981179
80.8k981179
add a comment |
add a comment |
$begingroup$
Use the formula $(fg)^{(n)}= sumlimits_{k=0}^{n} binom {n} {k} (f)^{(k)}(g)^{(n-k)}$.
$endgroup$
add a comment |
$begingroup$
Use the formula $(fg)^{(n)}= sumlimits_{k=0}^{n} binom {n} {k} (f)^{(k)}(g)^{(n-k)}$.
$endgroup$
add a comment |
$begingroup$
Use the formula $(fg)^{(n)}= sumlimits_{k=0}^{n} binom {n} {k} (f)^{(k)}(g)^{(n-k)}$.
$endgroup$
Use the formula $(fg)^{(n)}= sumlimits_{k=0}^{n} binom {n} {k} (f)^{(k)}(g)^{(n-k)}$.
answered 21 hours ago
Kavi Rama MurthyKavi Rama Murthy
54.1k32055
54.1k32055
add a comment |
add a comment |
$begingroup$
I get $f^{10}(x)=32e^xcos x$.
Here's what I did:
begin{align}f'(x)&=e^x(sin x+cos x)\
implies f''(x)&=2e^xcos x\
implies f^3(x)&=2e^x(cos x-sin x)\
implies f^4(x)&=2e^x(-2sin x)=-4f(x)\
implies f^5(x)&=-4f'(x)\
implies f^8(x)&=-4f^4(x)=16f(x)\
implies f^{10}(x)&=16f''(x)end{align}
$endgroup$
$begingroup$
Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
$endgroup$
– AlexanderJ93
21 hours ago
$begingroup$
Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
$endgroup$
– Rhys Hughes
21 hours ago
$begingroup$
@RhysHughes &Alexander J93 thanks
$endgroup$
– Chris Custer
21 hours ago
add a comment |
$begingroup$
I get $f^{10}(x)=32e^xcos x$.
Here's what I did:
begin{align}f'(x)&=e^x(sin x+cos x)\
implies f''(x)&=2e^xcos x\
implies f^3(x)&=2e^x(cos x-sin x)\
implies f^4(x)&=2e^x(-2sin x)=-4f(x)\
implies f^5(x)&=-4f'(x)\
implies f^8(x)&=-4f^4(x)=16f(x)\
implies f^{10}(x)&=16f''(x)end{align}
$endgroup$
$begingroup$
Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
$endgroup$
– AlexanderJ93
21 hours ago
$begingroup$
Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
$endgroup$
– Rhys Hughes
21 hours ago
$begingroup$
@RhysHughes &Alexander J93 thanks
$endgroup$
– Chris Custer
21 hours ago
add a comment |
$begingroup$
I get $f^{10}(x)=32e^xcos x$.
Here's what I did:
begin{align}f'(x)&=e^x(sin x+cos x)\
implies f''(x)&=2e^xcos x\
implies f^3(x)&=2e^x(cos x-sin x)\
implies f^4(x)&=2e^x(-2sin x)=-4f(x)\
implies f^5(x)&=-4f'(x)\
implies f^8(x)&=-4f^4(x)=16f(x)\
implies f^{10}(x)&=16f''(x)end{align}
$endgroup$
I get $f^{10}(x)=32e^xcos x$.
Here's what I did:
begin{align}f'(x)&=e^x(sin x+cos x)\
implies f''(x)&=2e^xcos x\
implies f^3(x)&=2e^x(cos x-sin x)\
implies f^4(x)&=2e^x(-2sin x)=-4f(x)\
implies f^5(x)&=-4f'(x)\
implies f^8(x)&=-4f^4(x)=16f(x)\
implies f^{10}(x)&=16f''(x)end{align}
edited 11 hours ago
lioness99a
3,6662727
3,6662727
answered 21 hours ago
Chris CusterChris Custer
11.3k3824
11.3k3824
$begingroup$
Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
$endgroup$
– AlexanderJ93
21 hours ago
$begingroup$
Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
$endgroup$
– Rhys Hughes
21 hours ago
$begingroup$
@RhysHughes &Alexander J93 thanks
$endgroup$
– Chris Custer
21 hours ago
add a comment |
$begingroup$
Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
$endgroup$
– AlexanderJ93
21 hours ago
$begingroup$
Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
$endgroup$
– Rhys Hughes
21 hours ago
$begingroup$
@RhysHughes &Alexander J93 thanks
$endgroup$
– Chris Custer
21 hours ago
$begingroup$
Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
$endgroup$
– AlexanderJ93
21 hours ago
$begingroup$
Your calculation for $f^{(4)}$ is wrong. Looks like you missed a product rule.
$endgroup$
– AlexanderJ93
21 hours ago
$begingroup$
Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
$endgroup$
– Rhys Hughes
21 hours ago
$begingroup$
Be careful: $$(e^x(g(x))'=e^xg(x)+e^xg'(x)$$ You missed the first bit in your calculation for $f^4$
$endgroup$
– Rhys Hughes
21 hours ago
$begingroup$
@RhysHughes &Alexander J93 thanks
$endgroup$
– Chris Custer
21 hours ago
$begingroup$
@RhysHughes &Alexander J93 thanks
$endgroup$
– Chris Custer
21 hours ago
add a comment |
$begingroup$
Regarding the question about looking for a pattern:
Repeated application of the product rule gives
$$ f(x) = e^x sin x$$
$$ f'(x) = e^x sin x + e^x cos x$$
$$ f''(x) = e^x sin x + e^x cos x + e^x cos x - e^x sin x = 2 e^x cos x$$
Is a pattern emerging?
$$ f^{(3)}(x) = 2 e^x cos x -2 e^x sin x$$
$$ f^{(4)}(x) = 2 e^x cos x -2 e^x sin x - 2 e^x sin x - 2 e^x cos x = -4 e^x sin x$$
Yes. We can then conclude that
$ f^{(6)}(x) = -8 e^x cos x$, $f^{(8)}(x) = 16 e^x sin x$, and $f^{(10)}(x) = 32 e^x cos x$
such that $f^{(10)}(0) = 32 $
$endgroup$
add a comment |
$begingroup$
Regarding the question about looking for a pattern:
Repeated application of the product rule gives
$$ f(x) = e^x sin x$$
$$ f'(x) = e^x sin x + e^x cos x$$
$$ f''(x) = e^x sin x + e^x cos x + e^x cos x - e^x sin x = 2 e^x cos x$$
Is a pattern emerging?
$$ f^{(3)}(x) = 2 e^x cos x -2 e^x sin x$$
$$ f^{(4)}(x) = 2 e^x cos x -2 e^x sin x - 2 e^x sin x - 2 e^x cos x = -4 e^x sin x$$
Yes. We can then conclude that
$ f^{(6)}(x) = -8 e^x cos x$, $f^{(8)}(x) = 16 e^x sin x$, and $f^{(10)}(x) = 32 e^x cos x$
such that $f^{(10)}(0) = 32 $
$endgroup$
add a comment |
$begingroup$
Regarding the question about looking for a pattern:
Repeated application of the product rule gives
$$ f(x) = e^x sin x$$
$$ f'(x) = e^x sin x + e^x cos x$$
$$ f''(x) = e^x sin x + e^x cos x + e^x cos x - e^x sin x = 2 e^x cos x$$
Is a pattern emerging?
$$ f^{(3)}(x) = 2 e^x cos x -2 e^x sin x$$
$$ f^{(4)}(x) = 2 e^x cos x -2 e^x sin x - 2 e^x sin x - 2 e^x cos x = -4 e^x sin x$$
Yes. We can then conclude that
$ f^{(6)}(x) = -8 e^x cos x$, $f^{(8)}(x) = 16 e^x sin x$, and $f^{(10)}(x) = 32 e^x cos x$
such that $f^{(10)}(0) = 32 $
$endgroup$
Regarding the question about looking for a pattern:
Repeated application of the product rule gives
$$ f(x) = e^x sin x$$
$$ f'(x) = e^x sin x + e^x cos x$$
$$ f''(x) = e^x sin x + e^x cos x + e^x cos x - e^x sin x = 2 e^x cos x$$
Is a pattern emerging?
$$ f^{(3)}(x) = 2 e^x cos x -2 e^x sin x$$
$$ f^{(4)}(x) = 2 e^x cos x -2 e^x sin x - 2 e^x sin x - 2 e^x cos x = -4 e^x sin x$$
Yes. We can then conclude that
$ f^{(6)}(x) = -8 e^x cos x$, $f^{(8)}(x) = 16 e^x sin x$, and $f^{(10)}(x) = 32 e^x cos x$
such that $f^{(10)}(0) = 32 $
answered 1 hour ago
Terje D.Terje D.
1213
1213
add a comment |
add a comment |
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