先化简再求值：设：A=2x^2-3xy+y^2-x+2y B=4x^2-6xy+2y^2-3x-y ,若/x-3a/+(

来源：学生作业帮编辑：拍题作业网作业帮分类：数学作业时间：2024/05/18 20:51:14

先化简再求值：设：A=2x^2-3xy+y^2-x+2y B=4x^2-6xy+2y^2-3x-y ,若/x-3a/+(y+5)^2=0,且B-2A=a;求A的

∵|x-3a|≥0,(y+5)^2≥0
又：|x-3a|+(y+5)^2=0
∴|x-3a|=(y+5)^2=0
∴x=3a,y=-5
a=B-2A= 4x^2-6xy+2y^2-3x-y -2(2x^2-3xy+y^2-x+2y )
= 4x^2-6xy+2y^2-3x-y -4x^2+6xy-2y^2+2x-4y
= -x-5y
x=3a=-3x-15y
x=-15y/2=-15*(-5)/2=75/2
A = 2x^2-3xy+y^2-x+2y
= (2x-y)(x-y)-x+2y
= （75+5)(75/2+5)-75/2+2*(-5)
= 80*85/2 - 75/2-10
= 340-38+1/2-10
= 292又1/2

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