# Fourier Series

[SOLVED] Fourier Series

1. Homework Statement
I know this may be a simple problem but im just beginning to understand this subject and this question has confused me.

Expand the following as a whole-range fourier series:

f(x) = 1 -pi < x < 0
f(x) = x 0 < x < pi

I can solve FS with one eqn, but what do you do with two, do you integrate twice or something?

2. Homework Equations

3. The Attempt at a Solution

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Hootenanny
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Welcome to PF Shomy,

The first thing to decide is whether the function is even, odd or neither.

Thanks for the welcome!
The first part is even and the second is odd

Hootenanny
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The first part is even and the second is odd
I suspect that the function is written thus,

$$f(x) = \left\{\begin{array}{cr}1 & -\pi<x<0 \\ x & 0<x<\pi\end{array}\right.$$

Which means it isn't actually two functions, but one piecewise function.

Yeah

Hootenanny
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So now, is the function odd or even?

The first part is even and the second is odd

Hootenanny
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The first part is even and the second is odd
So overall the function is...?

Odd?

Hootenanny
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Correct! What does this tell you about the Fourier series coefficients?

One of the coefficients (cos) is zero

Hootenanny
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One of the coefficients (cos) is zero
Correct, all the coefficients of cosine are zero. So what is the Euler formula for calculating the coefficients of sine?

Last edited:
Well i meant that one of the coefficients (ie the cos ) is zero because i am using sum notation.

an = 1/pi integral f(x)cosmx dx from pi to -pi. Do I integrate both functions and sum them or what?

Hootenanny
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Haven't we just agreed that the cosine coefficients are zero?

Yeah sorry

Hootenanny
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Yeah sorry
No problem . So what about the coefficients of sine?

bm = 1/pi integral f(x)sinmx dx

So do you integrate the first part from pi to 0 and the second part from 0 to -pi

Hootenanny
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So do you integrate the first part from pi to 0 and the second part from 0 to -pi
I believe it's the other way around. Overall the coefficient is given by,

$$b_n=\frac{1}{\pi}\int_{0}^{2\pi}f(x)\sin(nx)dx$$

However, since f(x) is defined piecewise we must write,

$$b_n = \frac{1}{\pi}\left[\int_{-\pi}^{0}1\cdot\sin(nx)dx + \int_{0}^{\pi}x\cdot\sin(nx)dx\right]$$

Is that what you meant?

Yeah thats what was holding up. Thanks so much!

Hootenanny
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Yeah thats what was holding up. Thanks so much!
No problem Redbelly98
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Homework Helper
> "Odd?"

Correct! What does this tell you about the Fourier series coefficients?
If I understand the function definition, I have a problem with this answer. For example consider x = 2:

f(2) = 2
f(-2) = 1

So for x = 2,
f(x) is not equal to f(-x)
and
f(x) is not equal to -f(-x)

What does this tell us about odd vs. even vs. neither?

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Hootenanny
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Good catch Redbelly! Of course the function is neither odd nor even, I really don't know what I was thinking ! I'll PM Shomy now and hopefully they haven't handed the assignment in.

I've been making a few daft mistakes recently Last edited:
Thank You!

Redbelly98
Staff Emeritus
I've been making a few daft mistakes recently It only gets worse as you get older 