Difference between revisions of "HWK11MA527Fall2010" - Rhea

Revision as of 16:16, 7 November 2010

Homework 11 collaboration area

Question: p. 499, #10: It says to convert it to real form, but when I use Euler's formula, I'm getting that there is still both a complex part and a real part for the Fourier series. Am I just supposed to write the real part, or am I doing this problem incorrectly? Thank you!

Answer: If you do it right, the imaginary part should be zero. Look for cancellation when you change cos(-A) to cos(A) and sin(-A) to -sin(A).

Question: I'm having trouble getting HWK 11, Page 499, Problem 3 started.

Answer: You will need to use Euler's identity

$ e^{i\theta}=\cos\theta+i\sin\theta $

and separate the definitions of the complex coefficients into real and imaginary parts. For example,

$ c_n=\frac{1}{2L}\int_{-L}^L f(x)e^{-inx}\,dx= $

$ =\frac{1}{2L}\int_{-L}^L f(x)(\cos(-nx)+i\sin(-nx))\,dx= $

$ =\frac{1}{2L}\int_{-L}^L f(x)(\cos(nx)-i\sin(nx))\,dx= $

$ =\frac{1}{2L}(\int_{-L}^L f(x)(\cos(nx)\,dx - i\int_{-L}^L f(x)\sin(nx)\,dx)= $

$ =\frac{1}{2}(a_n-ib_n). $

Do the same thing for $ c_{-n} $ and combine.

Question: Page 501 #3: (Example 1, really), What is $ C_n $? Is it the same $ C_n $ from the complex fourier series equation? If so, why have we discarded the negative n terms? Example 1 makes some really big algebraic leaps that I'm having trouble following. Can someone explain it more clearly?

Answer: It is a different $ C_n $:

$ C_n = \sqrt{A_n^2+B_n^2} $

where A_n and B_n are the coefficients gotten from doing the Method of Undetermined Coefficients with RHS (4/(n^2pi^2))cos(nt). See page 4 of

Bell's Lecture on Lesson 30

for the algebra. (Response): Thanks, because I wrote that lecture covered sections 11.5 and 11.6, I wasn't looking far forward enough in my notes. I see we did the whole thing out in class in my later pages.

Question: Page 506, Prob 15, if:

$ 2a_o=\frac{2\pi^4}{9} $

and

$ (a_n)^2=\frac{(4\pi^2)(cos)^2(nx)}{9} $

I dont understand where the $ \frac{\pi^4}{4} $ comes from? Can anyone point out what I am doing wrong?

Answer: Hmmm. Problem 15 on page 506 tells you to use Parseval's Identity applied to the function from problem 21 on page 485. The back of the book says that the Fourier Series for f(x)=x^2 between -pi and pi is

$ \frac{\pi^2}{3}-4\cos x + \frac{4}{2^2}\cos 2x-\frac{4}{3^2}\cos 3x + \cdots $

so it seems that

$ 2a_0^2=2\left(\frac{\pi^2}{3}\right)^2 $

and

$ a_n^2=\left(\frac{4}{n^2}\right)^2, $

and since all the b_n are zero, Parseval's Identity says

$ 2a_0^2+\sum_{n=1}^\infty a_n^2=\frac{1}{\pi}\int_{-\pi}^\pi|f(x)|^2\,dx. $

Question: Problem 1 on page 512: The answer key states that $ A=\frac{1}{(1+w^2)} $ but shouldn't it be $ A=\frac{2}{(1+w^2)} $ What am I missing here? Also, what are we supposed to do about the $ \frac{\pi}{2} $ and 0 terms? I see that they won't affect this problem being instantaneous and zero, respectively, but in general, what would we do if we had an evaluate-able expression for one interval, and then another nonzero expression for a following interval?

Answer: Since f(x)=0 for x<0, you don't multiply by 2 when changing limits from -inf..inf to 0..inf. That is only for even functions.

Questions: prob 11 on page 512:

Should I still use equation 10 to compute A(w) or should I use equation 12 to compute B(w) since f(x) is odd.

When I find A or B, what should the integral range be? (0 to pi?)

Answer: The function f is only defined for positive x. The Fourier Cosine Integral was cooked up by extending f to the negative real axis in such a way to make it an even function. That made the B(w) integral turn out to be zero.

Hence, you only need to calculate the A(w) integral in the form

A(w)= (2/pi) integral from 0 to infinity ...

Since f(x) is zero after pi, your integral would only really go from 0 to pi.

Is there a trick to work Problem #5 on page 505. I'm using the fact that abs(sin(x)) is an even function but for some reason the integration for An is not working. Anyone got any tips?

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