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当x=?12,y=?3时,求代数式3(x2-2xy)-[3x2-2y+...

原式=3x 2 -6xy-3x 2 +2y-2xy-2y=-8xy,当x= 1 2 ,y=-3时,原式=-12.

当 x=- 1 2 ,y=-3 时,原式= (- 1 2 ) 2 -2×(- 1 2 )×(-3)+(-3 ) 2 = 1 4 -3+9 = 6 1 4 .

因为 x+y=5, x^2+y^2=13 所以 2xy=(x+y)^2-(x^2+y^2) =5^2-13 =25-13 =12 xy=6 所以 x=2, y=3 或 x=3, y=2 所以 当 x=2, y=3 时, 当 x=3, y=2 时, x^2y+2xy^2+xy^2 x^2y+2xy^2+xy^2 =xy(x+2y+y) =xy(x+2y+y) =xy(x+y+y) =xy(x+y+y) =6X(5+3) =...

(2x3-3x2y-2xy2)-(x3-2xy2+y3)+(-x3+3x2y+y3)=2x3-3x2y-2xy2-x3+2xy2-y3-x3+3x2y+y3=0.∴不论x、y取什么值,代数式的值都为0.

原式=3x2y-2xy2+2(xy-32x2y)-xy+3xy2=3x2y-2xy2+2xy-3x2y-xy+3xy2=xy2+xy,当x=12,y=-13时,原式=-19.

∵x=2,y=-4,∴x+y=2-4=-2,x2+2xy+y2=(x+y)2=(-2)2=4.

化简得 原式等于-8xy值  -12

原式=3x2y-[2xy-2xy+3x2y+x2y2],=3x2y-2xy+2xy-3x2y-x2y2,=-x2y2,当x=2,y=-3时,原式=-22×(-3)2=-36.

2x-y=3 x-y/2=1.5 3-(x-y/2)=3-1.5 3-x+y/2=1.5

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