通分:x+y分之x-y,x²+xy分之x,x²-y²分之y

(x-y)/(x+y),x/(x²+xy)=1/(x+y),y/(x²-y²)=y/(x+y)(x-y)

分母的最小公倍数是(x+y)(x-y)通分得：
(x-y)/(x+y)=(x-y)²/(x+y)(x-y)
x/(x²+xy)=1/(x+y)=(x-y)/(x+y)(x-y)
y/(x²-y²)=y/(x+y)(x-y)

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再问： 已知a分之1-b分之1=2分之1,求a-b分之ab的值。
再答： 已知1/a-1/b=1/2 所以ab/(a-b)=1/(1/b-1/a)=1/(-1/2)=-2