Hi,(adsbygoogle = window.adsbygoogle || []).push({});

I am very new to Matlab, and I'm supposed to use the built-in FFT function to do discrete Fourier Transform for f(x) = sin x + 4 cos(5x) + (sin(6x))^2 on the interval [-pi, pi] with a uniform partition for the interval with n = 9. Then I have to

(a) Plot the magnitudes of the Fourier coefficients and

(b) Compute the first-order derivates at the grid points via FFT and compare them with f'(x).

Here's what I have for part (a):

x = -pi:0.25*pi:pi;

y = sin(x)+4*cos(5*x) + sin(6*x).*sin(6*x);

V=fft(y,9);

plot(abs(V));

I'm a little confused with what the function fft returns. Does it return the Fourier coefficients of f(x) in my program?

I got

V =

-0.0000

-5.9965 + 2.1842i

-4.5019 - 4.8898i

-8.3033 -15.3964i

0.8017 + 2.1116i

0.8017 - 2.1116i

-8.3033 +15.3964i

-4.5019 + 4.8898i

-5.9965 - 2.1842i

I also don't know how to find the first-order derivates at the grid points via FFT for part (b). What function do I use?

Thank you very much!

Regards,

Rayne

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Matlab/Fast Fourier Transform

Can you offer guidance or do you also need help?

Draft saved
Draft deleted

**Physics Forums | Science Articles, Homework Help, Discussion**