(a) Sketch a graph of three cycles of the function with period \(2 \pi\) given by $$ f(t)=1-\frac{|t|}{\pi}, \quad-\pi \leq t<\pi $$ (b) Find its Fourier series representation.

Short Answer

Expert verified
(a) Sketch the function: Now sketching the first cycle, we have a straight line that starts at 1 when t = -π and decreases linearly to 0 at t = 0, then increases linearly back to 1 at t = π. For the second cycle, we have the same graph shifted to the right by 2π; this means that the graph in this cycle starts at 1 when t = π and decreases linearly to 0 at t = 2π, then increases linearly back to 1 at 3π. And for the third cycle, the graph is the same as the first cycle shifted to the left by 2π; this means that the graph starts at 1 when t = -3π and decreases linearly to 0 at t = -2π, then increases linearly back to 1 at t = -π. Combining these three cycles, we get a periodic linear function consisting of the three cycles as explained. (b) Final Fourier series representation: After calculating a0 and an, let's say: $$ a_0 = A \\ a_n = B_n $$ Substituting these values, the Fourier series representation for the given function is as follows: $$f(t) = \frac{A}{2}+\sum_{n=1}^{\infty} B_n \cos(nt)$$ where A and Bn are the calculated values of the coefficients a0 and an, respectively.

Step by step solution

01

(a) Sketch the function

Since the function is given by \(f(t) = 1-\frac{|t|}{\pi}\) for \(-\pi \leq t < \pi\), we could graph these points within the given range of \(t\). To sketch three cycles, we need to extend the function outside its given range. The function is periodic with a period of \(2\pi\), so we could extend it by symmetry. The function for three cycles would look like this: $$ f(t)= \begin{cases} 1-\frac{|t+2\pi|}{\pi}, & -3\pi \leq t< -\pi\\ 1-\frac{|t|}{\pi}, & -\pi \leq t< \pi \\ 1-\frac{|t-2\pi|}{\pi}, & \pi \leq t < 3\pi \end{cases} $$ Now, you can sketch the graph of the function for each of the given ranges and combine them all, making sure to maintain the necessary symmetries.
02

(b) Find the Fourier series representation

For a function \(f(t)\) with period \(2\pi\), the Fourier series representation is given by: $$f(t) = \frac{a_0}{2} + \sum_{n=1}^{\infty} [a_n \cos(nt) + b_n \sin(nt)]$$ where the coefficients \(a_0, a_n,\) and \(b_n\) are calculated as follows: $$a_0 = \frac{1}{\pi}\int_{-\pi}^{\pi} f(t) dt$$ $$a_n = \frac{1}{\pi}\int_{-\pi}^{\pi} f(t) \cos(nt) dt$$ $$b_n = \frac{1}{\pi}\int_{-\pi}^{\pi} f(t) \sin(nt) dt$$ The integral limits are set as \([-\pi, \pi]\) because the function has a period of \(2\pi\).
03

Calculate the coefficients

First, let's find the \(a_0, a_n\) and \(b_n\) coefficients. \(a_0 = \frac{1}{\pi}\int_{-\pi}^{\pi} (1-\frac{|t|}{\pi}) dt\) Since \(f(t)=1-\frac{|t|}{\pi}\) is an even function, \(b_n = 0\). Thus, we only need to calculate \(a_n\): \(a_n = \frac{1}{\pi}\int_{-\pi}^{\pi} (1-\frac{|t|}{\pi}) \cos(nt) dt\) By calculating these coefficients, we can represent the Fourier series for the given function.
04

Final Fourier series representation

After calculating \(a_0, a_n\) and finding that \(b_n=0\) for the given function, you can write the Fourier series representation as: $$f(t) = \frac{a_0}{2} + \sum_{n=1}^{\infty} a_n \cos(nt)$$ Substitute the calculated values for \(a_0\) and \(a_n\) to obtain the final solution.

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