设<imgwidth="35"height="21"v:shapes="_x0000_i1063"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image070.png">为随机变量,且<imgwidth="223"height="21"v:shapes="_x0000_i1064"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image072.png">则<imgwidth="84"height="21"v:shapes="_x0000_i1065"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image046.png">()