1. 设题4图 (a)所示信号<img src="https://static.zxtiku.com/wangke/imgs/topic/8d5c8d53241dfc03165719bb0e442541.png"/>的频谱<img src="https://static.zxtiku.com/wangke/imgs/topic/7cd383d71e1219e20ccc2ea789781dd7.png"/>已知，则题4图 (b)所示信号<img src="https://static.zxtiku.com/wangke/imgs/topic/a80060317d0b01eb8ea7695b808ca448.png"/>的频谱<img src="https://static.zxtiku.com/wangke/imgs/topic/64f4cb8bdb33b666bf0183e4cf0fc3c6.png"/> 为 ( )<br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/6ce5a23bd8656bad44e448773f86e017.jpg"/>