((x+y)/(x+y))的平方乘(2y-2x)/(3x+3y)-(x的平方)/(x的平方-y的平方)除以x/y
=(x+y)²/(x-y)² 乘2(y-x)/3(x+y)-x²/(x+y)(x-y)乘y/x
=-2(x+y)/3(x-y) - xy/(x+y)(x-y)
=[-2(x+y)²-3xy]/[3(x-y)(x+y)]
=(-2x²-2y²-7xy)/(3x²-3y²)