【x-y-(x-y分之4y平方)】【x+y-(x+y分之4x平方)】除以【3(x+y)-x-y分之8xy】
=（x-y-x+4y)(x+y-x-4x/y)/(3x+3y-x-8x)
=3y*(y-4x/y)/(3y-6x)
=(3y平方-12x)/(3y-6x)