已知x-xy=20,xy-y=-10,则x-y=?,x+y-2xy?
问题描述：
已知x-xy=20,xy-y=-10,则x-y=?,x+y-2xy?
答
两个方程设为 ①式和②式，然后 ①+②，得x-y=10，而 ①-②，得x+y-xy=30，还望楼主采纳，谢谢，
答
x-y=10
x+y-2xy=30
答
X-Y=X-XY+XY-Y
=20-10
=10
X+Y-2XY
=X-XY-(XY-Y)
=20-(-10)
=30
答
10
30