设随机变量<imgwidth="90"height="21"v:shapes="_x0000_i1067"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb08.files/image084.png">,则<imgwidth="77"height="21"v:shapes="_x0000_i1068"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb08.files/image086.png">()