先化简,再求值.4xy-[(x2+5xy-y2)-2(x2+3xy-1/2y2)],其中:x=-1,y=2.

问题描述:

先化简,再求值.4xy-[(x2+5xy-y2)-2(x2+3xy-

1
2
y2)],其中:x=-1,y=2.

原式=4xy-[x2+5xy-y2-2x2-6xy+y2]
=4xy-[-x2-xy]
=x2+5xy,
当x=-1,y=2时,
原式=x2+5xy
=(-1)2+5×(-1)×2
=-9.