Determine the frequency with find and diff

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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)