2分之1x的2次方-xy+2分之1y的2次方=x²/2-xy+y²/2=1/2(x²-2xy+y²)=1/2(x-y)²
=x²/2-xy+y²/2=1/2(x²-2xy+y²)=1/2(x-y)²嘻嘻 so easy！！！哎初二小儿科
