说明：  给定一个信号 a)画出低通信号的波形； b)画出抽样速率为 fs为 4HZ 的抽样序列 c)从抽样序列中恢复出原始信号 当抽样速率为 fs  1.5HZ ，画出恢复后的波形。 当抽样速率为 fs  1.5HZ ，画出恢复后的波形。
(Given a signal a) Draw a low pass signal waveform b) Draw the sampling rate fs is sampling sequence 4HZ c) From the sample sequence to recover the original signal When the sampling rate is fs  1.5HZ, waveform draw restored. When the sampling rate is fs  1.5HZ, waveform draw restored.)