先化简,再求值:[(x+y)(x-2y)-(x-2y)2]÷(-3y),其中x=-1,y=1/2.
问题描述:
先化简,再求值:[(x+y)(x-2y)-(x-2y)2]÷(-3y),其中x=-1,y=
. 1 2
答
原式=[x2-2xy+xy-2y2-x2-4y2+4xy]÷(-3y)
=[3xy-6y2]÷(-3y)
=-x+2y,
当x=-1,y=
时,原式=1+2×1 2
=2.1 2