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+
xy)]−(4x2y+xy)}−7xy2,其中x=−1 2
,y=-16. 1 4
答
原式=5x2y+{xy-[5x2y-7xy2-
xy]-4x2y-xy}-7xy21 2
=5x2y+{xy-5x2y+7xy2+
xy-4x2y-xy}-7xy21 2
=5x2y+xy-5x2y+7xy2+
xy-4x2y-xy-7xy21 2
=
xy,1 2
当x=-
,y=-16,原式=1 4
×(-1 2
)×(-16)=2.1 4