已知,A=3x2+3y2-5xy,B=2xy-3y2+4x2,求: (1) 2A-B; (2)当x=3,y=−1/3时,2A-B的值.

问题描述:

已知,A=3x2+3y2-5xy,B=2xy-3y2+4x2,求:
(1) 2A-B;
(2)当x=3,y=−

1
3
时,2A-B的值.

(1)2A-B=2(3x2+3y2-5xy)-(2xy-3y2+4x2
=6x2+6y2-10xy-2xy+3y2-4x2
=2x2+9y2-12xy;
(2)当x=3,y=−

1
3
时,
2A-B=2x2+9y2-12xy=31.