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

原式=[x2-2xy+xy-2y2-x2-4y2+4xy]÷(-3y)
=[3xy-6y2]÷(-3y)
=-x+2y,
当x=-1,y=

1
2
时,原式=1+2×
1
2
=2.