You need to create a digital representation of the function. For example specify t then create F, then call FFT
t = 1 : 1000; % Whatever...
period = 360; % Whatever you want.
F = 1 + 2 * sin(2 * pi * t / period);
ft = fft(F); % FFT of entire signal.
subplot(1, 2, 1);
plot(real(ft), 'b-', 'LineWidth', 2);
grid on;
title('Real Part', 'FontSize', 20);
subplot(1, 2, 1);
plot(imag(ft), 'b-', 'LineWidth', 2);
grid on;
title('Imaginary Part', 'FontSize', 20);
The 5 you put in is not the number of harmonics like you're thinking about them. It's the number of elements in the transform. If you use 5 that may be too low resolution to even see your signal clearly and way too low to see your harmonics. Use more resolution and look for the spikes in the FFT signal. These will be at multiples of the (1/period) frequency.
