Question

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/+(

Answer 1

∵|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