整体代入，求代数式的值

Question

如题所述

Answer 1

(x+2y-3xy)-(-2x-y+xy),其中x+y=二分之一,xy=-二分之一用整体代入法求值.

解：原式=(x+2y-3xy)-(-2x-y+xy)

=x+2y-3xy+2x+y-xy
=3x+3y-4xy
=3*1/2-4*(-1/2)
=3/2+2
=7/2