二次型<imgwidth="390"height="28"v:shapes="_x0000_i1294"src="http://wljy.whut.edu.cn/uploadfiles/word/xxds05.files/image335.png">的标准型为()
①
<imgwidth="126"height="48"v:shapes="_x0000_i1295"src="http://wljy.whut.edu.cn/uploadfiles/word/xxds05.files/image337.png">
②
<imgwidth="116"height="48"v:shapes="_x0000_i1296"src="http://wljy.whut.edu.cn/uploadfiles/word/xxds05.files/image339.png">
③
<imgwidth="126"height="46"v:shapes="_x0000_i1297"src="http://wljy.whut.edu.cn/uploadfiles/word/xxds05.files/image341.png">
④
<imgwidth="126"height="48"v:shapes="_x0000_i1298"src="http://wljy.whut.edu.cn/uploadfiles/word/xxds05.files/image343.png">