设计低通IIR滤波器；并将此低通滤波器映射为高通、带通或带阻滤波器。并验证变换后的滤波器是否满足指标。并求输入x(n)=[cos(w1n)+cos(w2n)]u(n)经过系统后的输出y(n)-Low-pass IIR filter design　and mapping this low-pass filter high pass, band pass or band stop filter. And verify whether the filter transformation to meet the target. And to seek input x (n) = [cos (w1n)+ cos (w2n)] u (n) through the system output after the y (n)