先化简,后求值:5x2y+{xy−[5x2y−(7xy2+1/2xy)]−(4x2y+xy)}−7xy2,其中x=−1/4,y=-16.

问题描述:

先化简,后求值:5x2y+{xy−[5x2y−(7xy2+

1
2
xy)]−(4x2y+xy)}−7xy2,其中x=−
1
4
,y=-16.

原式=5x2y+{xy-[5x2y-7xy2-

1
2
xy]-4x2y-xy}-7xy2
=5x2y+{xy-5x2y+7xy2+
1
2
xy-4x2y-xy}-7xy2
=5x2y+xy-5x2y+7xy2+
1
2
xy-4x2y-xy-7xy2
=
1
2
xy,
当x=-
1
4
,y=-16,原式=
1
2
×(-
1
4
)×(-16)=2.