Homework 9 collaboration area
This is the place!
From Jake Eppehimer:
I am not sure how to do number 6 on p. 494. I'm clueless, and there's no answer in the back to verify if I'm doing anything right. Any tips?
From Shawn Whitman: The method of undetermined coefficients for second order nonhomogeneous linear ODEs works well for this problem. See pages 81-84 and use the sum rule. Two of the constants will go to zero. Two others will result in 1/(omega^2-alpha^2) and 1/(omega^2-beta^2); thus the given constraints.
From Mnestero:
I concur with Shawn regarding problem 6. I have a question about the even extension in problem 29. I am getting that the fourier series is 2/pi-4/pi(1/3 cos(2x) + 1/(3*5) cos (4x) + 1/(5*7) *cos(6x)... The answer in the book has odd numbers instead. Their answer doesn't make sense to me. Any thoughts?
From Shawn Whitman: My answer is also different than the one in the back of the book. an is 0 for odd n and -4/pi((n^2)-1) for even n. Note that the (n^2)-1 in the denominator is not a problem for n=1 if you start the summation from 2 instead of 1. This is okay since the odd n’s are zero. ((n^2)-1), for even n, yields 3, 15, 35 or 3*1, 3*5, 5*7 as the book suggests.
From Jayling:
I think the book answer is for Question 28 not 29. If you plot the function it looks like the sawtooth function. Anyway for the Fourier Sine Function did you guys get bn=0 for all n? Which implies that the Fourier Sine Function is equal to zero for all x.
From Jake Eppehimer:
Thank you! The reference page from 81-84 was very handy because I wasn't sure what to do since there were two sine functions added. I got what you got. And I am glad I'm not the only one having that exact issue on problem 29... So it's likely a book error? Jayling, I got that too, but according to the lecture, the odd expansion of sin(x) is simply sin(x), and it can be seen by graphing it. So that's what I'm going with.
