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

问题描述:

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

1
3
,y=
1
3

原式=x2+2xy+y2-3x2-9xy+2x2-8y2
=-7xy-7y2
当x=-

1
3
,y=
1
3
时,
-7xy-7y2
=-7(x+y)
=0.