化简求值：【(x+Y)(X-Y)-(X-Y)的平方+2Y（2-Y）】除4Y,其中X=1 Y=2
问题描述：
化简求值：【(x+Y)(X-Y)-(X-Y)的平方+2Y（2-Y）】除4Y,其中X=1 Y=2
答
(x-y)[(x+y)+x-y]+4y-2y^2
=2x(x-y)+4y-2y^2
=2[x^2-xy+2y-y^2]
所以 =2[1-2+4-4]/8=-1/4