Taylor series

uberathlete

New member
Joined
Jan 16, 2006
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48
Hi everyone. I'm having some problems with this question:

Find the Taylor series centered at Pi/4 for f(x) = sinxcosx

What I've done is find derivatives from orders 1 and up and evaluating them with x = Pi/4. I then used these on the general Taylor series formula to get an indication of how the series would look like then I converted it to a form using the summation sign. My answer basically ends up being:

1/2 - sum(from n= 1 to infinity) [(-1)^n * 2^(2n+1)*x^(2n)]/(n!)


I'm not so sure if this is the right answer, and I'm wondering if there's an easier way to to do this without having to manually calculating a whole lot deriveatives.

If anyone could help me out on this it'd be greatly appreciated. Thanks!
 
f(x)=sin(x)cos(x)
f'(x)=-sin²(x)+cos²(x)
f''(x)=-2sin(x)cos(x)-2cos(x)sin(x) =
-4sin(x)cos(x) =
-4*f(x)
so it repeats with the multiplier "exponentiating" and two sumations, even and odd.
I don't off hand see that it matches your answer but...
 
Have you noticed that sin(2x) = 2sin(x)cos(x)?
 
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