说明：  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)