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

问题描述:

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

1
2

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

1
2
时,原式=-10.