matlab 实现傅里叶变换 fs FT DFS DFT FFT 以及余弦函数的抽样 离散傅里叶变换 频谱 函数叠加后的傅里叶变换 频谱 -matlab to achieve the Fourier transform fs FT DFS DFT FFT and cosine functions sampling discrete Fourier transform the spectrum of the Fourier transform of the spectrum function superimposed