若x/y=-2,求x(2次)-2xy-3y(2次)/x(2次)-6xy-7y(2次)的值
要过程
解:(x²-2xy-3y²)/(x²-6xy-7y²)=【(x+y)(x-3y)】/【(x+y)(x-7y)】=(x-3y)/(x-7y)(注:就是配方)
∵x/y=-2
∴x= -2y
把x= -2y带入
原式= (-2y-3y)/(-2y-7y)=(-5y)/(-9y)=5/9