设<imgwidth="35"height="21"v:shapes="_x0000_i1104"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image133.png">是相互独立且均服从正态分布<imgwidth="54"height="41"v:shapes="_x0000_i1105"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image135.png">的随机变量,则<imgwidth="88"height="21"v:shapes="_x0000_i1106"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image137.png">()
①
<imgwidth="29"height="44"v:shapes="_x0000_i1107"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image139.png">
②
<imgwidth="29"height="44"v:shapes="_x0000_i1108"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image139.png">
③
<imgwidth="28"height="46"v:shapes="_x0000_i1109"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image141.png">
④
<imgwidth="29"height="46"v:shapes="_x0000_i1110"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb09.files/image143.png">