# please solve a 2013 th derivative question?

$f(x) = 6x^7\sin^2(x^{1000}) e^{x^2}$

Find $f^{(2013)}(0)$

A math forum friend suggest me to use big O symbol, however have no idea what that is, so how does that helping?

• Where did you find this problem? 2013 is an odd year to choose... :) – apnorton Dec 30 '12 at 20:58
• $\sin^2(x^{1000}) = x^{2000}+O(x^{4000})$ reduces the complexity of the computation considerably... – WimC Dec 30 '12 at 21:04

Note that,

$$6\,x^{7} \sin\left(x^{1000}\right)\sin\left(x^{1000}\right)e^{x^2}$$

$$= 6\,x^{7} \left( x^{1000}-\frac{x^{3000}}{3!}+\dots \right)\left( x^{1000}-\frac{x^{3000}}{3!}+\dots \right)\left(1+\frac{x^2}{1!}+\frac{x^4}{4!}+\dots\right)$$

$$= 6x^7x^{2000}\left( 1-\frac{x^{2000}}{3!} +\dots\right)^2\left(1+\frac{x^2}{1!}+\frac{x^4}{2!}+\dots\right)$$

$$= 6x^{2007}\left(1+\frac{x^2}{1!}+\frac{x^4}{2!}+\frac{x^6}{3!}+\dots\right)\left( 1-\frac{x^{2000}}{3!} +\dots\right)^2$$

Now, it is clear that the coefficient of $x^{2013}$ is $1$, which implies that

$$\frac{f^{(2013)}(0)}{(2013)!} = 1 \implies f^{(2013)}(0)=(2013)!.$$

• Nice job there (+1) – user 1591719 Dec 31 '12 at 20:56
• @Chris'ssister: Thank you. – Mhenni Benghorbal Dec 31 '12 at 23:08
• Best answer as usual.Mhenni – Aryadeva Sep 30 '17 at 16:29

Hint:

Consider the Taylor expansion of $f$

• Short and sweet...+1 :-) – amWhy Dec 30 '12 at 21:14