(8xy-3x2)-5xy-3(xy-2x2+3)已知A=x3+x2-x+1,B=-x3-x2+x+1,当x=-2时,求A-2B的值.

分类: 作业答案 • 2022-07-14 19:20:22

问题描述：

(8xy-3x2)-5xy-3(xy-2x2+3)
已知A=x3+x2-x+1,B=-x3-x2+x+1,当x=-2时,求A-2B的值.

答

①原式=8xy-6-5xy-3xy+12-9=-3
②∵A=x3+x2-x+1
∴A=4X+1
∵B=-x3-x2+x+1
∴B=-4X+1
∴A-2B=4X+1-2（-4X+1）=12X-1
当X=-2时，12X-1=12×（-2）-1=-25

答

(1)(8xy-3x2)-5xy-3(xy-2x2+3)=8xy-3x2-5xy-3xy+6x2-9=3x2-9=3*4-9=3
(2)A=-8+4+2+1=-1
B=8-4-2+1=3
A-2B=-1-2*3=-7

答

A=-6-4+2+1=-7,B=6+4-2+1=9
A-2B=-7-2x9=-25

答

(8xy-3*2)-5xy-3(xy-2*2+3)=8xy-6-5xy-(3xy-12+9)=-6+12-9=-3
A-2B=x3+x2-x+1-2(-x3-x2+x+1)=3x3+3x2-3x-1=3(x3+x2-x)-1
当x=-2时，即3*（-8+4+2）-1=-7