x的平方-y的平方分之2xy + x+y分之x - y-x分之y

（x的平方-y的平方)分之2xy + (x+y)分之x - (y-x)分之y=（x的平方-y的平方)分之2xy + (x+y)分之x + (x-y)分之y=(x²-y²)分之[2xy+x(x-y)+y(x+y)]=(x²-y²)分之(2xy+x²-xy+yx+y²)=(x²-y²)分之(2xy+x²+y²)=[(x+y)(x-y)]分之(x+y)²=(x-y)分之(x+y)