（1）1/2a²+ab+1/2b² =1/2(a²+2ab+b²)=1/2(a+b)²（2）a²(x-y)+b²（y-x) =a²(x-y)-b²（x-y)=(x-y)(a²-b²）=(x-y)(a+b)(a-b)(3)（x+y)²-4xy=x²+2xy...