= np.arange(0, 10, 0.1)
signal = np.sin(2 * np.pi * 1.0 * t) + 0.5 * np.sin(2 * np.pi * 3.0 * t)
fft_result = np.fft.fft(signal)
freqs = np.fft.fftfreq(len(t), d=0.1)

Why does the peak show up at index 10 and 30?