5x^2-3xy-4y^2-11xy-7x^2+2y^2
=-2x^2-14xy-2x^2
=-x^2-7xy-x^2
=x^2+7xy+x^2
=（x+y）^2+5xy
=0a＜b＜0＜c化简|a|-|a+b|+|c-a|+|b+c|