答:
x²+y²-4x+6y+13=0
(x²-4x+4)+(y²+6y+9)=0
(x-2)²+(y+3)²=0
所以:
x-2=0
y+3=0
解得:x=2,y=-3
所以:
(3x+y)²-3(3x-y)(x+y)-(x-3y)(x+3y)
=9x²+6xy+y²-3(3x²+2xy-y²)-(x²-9y²)
=9x²+6xy+y²-9x²-6xy+3y²-x²+9y²
=13y²-x²
=13×9-4
=117-4
=113
追问为何4x要加4,6y的就加九?
追答凑成完全平方的形式啊....