若A=x2-2xy+1/3y2,B=2x-3xy+y2,其中x=1,y=-2,求2A-B的值_数学解答

若A=x2-2xy+1/3y2,B=2x-3xy+y2,其中x=1,y=-2,求2A-B的值

人气：470 ℃　时间：2019-11-07 07:51:50

解答

2A-B
=2(x2-2xy+1/3y2)-(2x-3xy+y2)
=2x2-4xy+2/3y2-2x+3xy-y2
=2x2-xy-y2+2/3y2
=2*1^2-1*(-2)-(-2)2+2/3(-2)2
=2+2-4+1/6
=1/6