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xy y 2xy 3x y y 2y 1

2A-3B =2(3x²-xy+y²)-3(2x²-3xy-2y²) =6x²-2xy+2y²-6x²+9xy+6y² =7xy+8y² 当x=1/7,y=1/8时 原式=7×1/7×1/8+8×1/8×1/8 =1/8+1/8 =1/4

解: 1/x -1/y=2 (y-x)/(xy)=2 y-x=2xy x-y=-2xy (2x-3xy-2y)/(3x+4xy-3y) =[2(x-y)-3xy]/[3(x-y)+4xy] =[2·(-2xy)-3xy]/[3·(-2xy)+4xy] =(-7xy)/(-2xy) =7/2

一3x²y+2X²y+3xy²一2xy² =-x²y+xy² =-1²x5+1x5² =-5+25 =20

2(3x2-2xy+4y2)-3(2x2-xy+2y2),=6x2-4xy+8y2-(6x2-3xy+6y2),=6x2-4xy+8y2-6x2+3xy-6y2,=-xy+2y2;当x=2,y=-1时,原式=-2×(-1)+2×(-1)2=2+2=4.

解:已知x分之1-y分之1=2, 得xy分之y-xy分之x=2 xy分之(y-x)=2 y-x=2xy x-y=-2xy 得(x-xy-y)分之(3x-2xy-3y) =[(x-y)-xy]分之[(3x-3y)-2xy] =[(x-y)-xy]分之[3×(x-y)-2xy] =(-2xy-xy)分之[3×(-2xy)-2xy] =(-3xy)分之(-6xy...

(1)原式=4a2+18a-9a2-12a+3=-5a2+6a+3;(2)原式=3x2-6xy-9x2+6y-6xy-6y=-6x2-12xy,当x=12,y=-34时,原式=-32-6×34=-20512.

同学,高数大一的吧,哈哈,我去年也做过

(1)原式=-2x-4x+2y+3x-2y+1=-3x+1,当x=-3时,原式=10;(2)原式=xy-2y2-8xy+6y2-2x2y+30y2-4x2y=-7xy+34y2-6x2y,当x=1,y=-2时,原式=162.

原式=3xy 2 -2xy+3x 2 y+3x 2 y-2xy 2 =xy 2 -2xy+6x 2 y,当x=-4,y= 1 2 时,原式=-4×( 1 2 ) 2 -2×(-4)× 1 2 +6×(-4) 2 × 1 2 =-1+4+48=52.

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