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

问题描述:

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=-5a=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-5yx=3a=-3x-15yx=-15y/2=-15*(-5)/...