So for each frequency, we actually need to multiply by a sine wave and a cosine wave and find the amplitudes for each.

因此，对于每个频率，我们实际上需要乘以正弦波和余弦波并找到每个频率的振幅。

You multiply each point by a sine wave of this frequency and a cosine wave of this frequency and separately add them up.

将每个点乘以该频率的正弦波和该频率的余弦波，然后将它们分别相加。

Now so far I've only talked about sine waves but if the signal is a cosine wave, then even if you multiply it by a sine wave of the exact same frequency, the area under the curve will be zero.

到目前为止， 我只讨论了正弦波， 但如果信号是余弦波，那么即使将其乘以完全相同频率的正弦波，曲线下的面积也将为零。