说明：  用振幅为0.8的方波进行傅立叶分析，并用分析得到的系数求解当K取不同值时的合成信号
( F = FOURIER(f) is the Fourier transform of the sym scalar f with default independent variable x. The default return is a function of w. This example is useful for the persons who learn sth about signal processing.)