题26图所示的离散时间LTI系统的系统函数<img src="https://static.zxtiku.com/wangke/imgs/topic/2aae3426658225fc0df12da592a827f8.png"/>是 ( )<br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/354fe7754152dc85b5cfcce9297db6a0.jpg"/>