因式分解：1/2xy-1/2y^2-(x-y)^2

1/2xy-1/2y^2-(x-y)^2
=1/2xy-1/2y²-x²+2xy-y²
=-x²+5/2xy-3/2y²
=-1/2(2x²-5xy+3y²)
=-1/2(x-y)(2x-3y)
2x(3x-1)+(2x-2)(1-3x)
=2x(3x-1)-(2x-2)(3x-1)
=(3x-1)(2x-2x+2)=6x-2
6X-2=28 x=5
已知1+x+x^2+x^3=0
1+x+x^2+.+x^2011
=1+x+x^2+x^3+x^4(1+x+x^2+x^3)+.+x^2008(1+x+x^2+x^3)
=0+0+.+0
=0
设X=9998,
原式=(X^3-2X^2-X+2)/(X^3+X^2-X-1)
=［X^2(X-2)-(X-2)］/［X^2(X+1)-(X-1)］
=(X-2)(X^2-1)/［(X+1)(X^2-1)］
=(X-2)/(X+1)
=9996/9999
=3332/3333

1/2xy-1/2y^2-(x-y)^2
=1/2xy-1/2y²-x²+2xy-y²
=-x²+5/2xy-3/2y²
=-1/2(2x²-5xy+3y²)
=-1/2(x-y)(2x-3y)

1/2y(x-y)-(x-y)^2
=(x-y)[(1/2)y-(x-y)]
=(1/2)(x-y)(y-2x)

1/2xy-1/2y²-(x-y)²
=1/2y(x-y)-(x-y)²
=(x-y)[1/2y-(x-y)]
=(x-y)(3/2y-x)

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(3/2y-x)(x-y)