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

问题描述:

先化简,再求值:2x2+(-x2+3xy+2y2)-( x2-xy+2y2),其中 x=

1
2
,y=3.

原式=2x2-x2+3xy+2y2-x2+xy-2y2
=(2-1-1)x2+(3+1)xy+(2-2)y2
=4xy,
当x=

1
2
,y=3时,原式=4×
1
2
×3=6.