题目有误,改为“已知X-3Y=0,求（2X+Y)(X-Y)/(X^2-2XY+Y^2）的值
（2X+Y)(X-Y)/(X^2-2XY+Y^2）
=（2X+Y)(X-Y)/(X-Y)^2
=（2X+Y)/(X-Y)
已知X-3Y=0得X=3Y带入上式得：
（2X+Y)(X-Y)/(X^2-2XY+Y^2）
=（2X+Y)/(X-Y)
=(6Y+Y)/(3Y-Y)
=7/2