For problem #18, I can take the derivative of the sum ok, but then am stuck when it comes to actually figuring out what it converges to as n > inf. Here's what I get for f':
$$$f'(x) = \sum_{n=0}^\infty (1)^n (2n) x^{2n1}$$$
Any help appreciated...thanks in advance! =)
GRE 9367 #18
Re: GRE 9367 #18
think abou the geometric series $$\frac{1}{1+x^{2}}=\sum_{n\geq 0}(1)^{n}x^{2n}$$
now take the derivative both sides !
now take the derivative both sides !

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 Joined: Sat Aug 08, 2009 5:55 pm
Re: GRE 9367 #18
Ahalesson learned to not psyche myself out before trying to manipulate a sum into a familiar form...
Thanks!
Thanks!