Question

数与式的计算X分之一减去Y分之一等于2,则X-XY-Y分之3X+XY-3Y的值为?

Answer 1

1/x-1/y=2,通分后就是（y-x）/xy=2,所以y-x=2xy,x-y=-2xy

然后再带入到题目中

X-XY-Y/(3X+XY-3Y)=[(X-Y)-XY]/[3(X-Y)+XY]

=[(-2XY)-XY]/[3*(-2XY)+XY]=-3XY/(-5XY)=3/5