2x-y分之x+y=2,则2x-y=2(x+y)
4x-2y分之x+y-4x+4y分之2x-y
=2(2x-y)分之x+y-4(x+y)分之2x-y
=4(x+y)分之x+y-4(x+y)分之2(x+y)
=4分之1+4分之2
=4分之3