输入信号的时域和频谱分析

输入信号为ft=[5cos(600*pi*t)+4cos(90*pi*t)+9]/18

输入信号为高频和低频信号的叠加：高频300Hz，低频45Hz-　Of the input signal in time domain and spectral analysis　of the input signal is ft = [5cos (600* pi* t)+4 cos (90* pi* t)+9]/18　input signal for the high-frequency and low frequency signal superimposed: frequency 300Hz, 45Hz low frequency