Determine the frequency with find and diff

I have the problem to find the frequencies where peaks occur. Here is the question:

Here is my solution for part a and I need to find out frequencies at which the peaks occur. Thanks in advance for your help. I gave a try a bit for part b but still could't get the result.

% Question 3

N = 2^10;
delta_t = 2*pi/4*40*pi;
%finding Period T from N - number of cycles, delta_t - change of time
T = delta_t/N
%part a
n = [1 2 3 4]
omega = [10*pi 18*pi 18.8*pi 40*pi]
fi = [0.1 0.04 0.04 0.03]
H = [1 1.3 1.3 1.8]
syms n
f1 = symsum(exp(-fi.*omega*t).*sin(sqrt(1-(fi).^2).*omega*t),n, 1, 4)
subplot(1,2,1)
fplot(f1)
f2 = symsum(H.*exp(-fi.*omega*t).*sin(sqrt(1-(fi).^2).*omega*t),n, 1, 4)
subplot(1,2,2)
fplot(f2)

Answer 1

[Ameer Hamza]

You can use findpeaks(): https://www.mathworks.com/help/signal/ref/findpeaks.html or islocalmax(): https://www.mathworks.com/help/matlab/ref/islocalmax.html

[Elnur Shahbalayev]

Thank you so much for your answer. I tried them before but couldn't get answer. If possible please can you show with code? Thanks in advance again!