设二维随机变量<imgwidth="46"height="21"v:shapes="_x0000_i1049"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image042.png"><imgwidth="79"height="41"v:shapes="_x0000_i1050"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image044.png">,则<imgwidth="84"height="21"v:shapes="_x0000_i1051"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image046.png">()
①
<imgwidth="16"height="41"v:shapes="_x0000_i1052"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image048.png">