通分x/x^2-1与2/x^2+2x+1

1.
最简公分母是(x+1)^2(x-1)
x/(x^2-1)=x(x+1)/(x+1)^2(x-1)
2/(x^2+2x+1)=2(x-1)/(x+1)^2(x-1)
2.
最简公分母是x(x+y)(x-y)
1/(x^2-y^2)=x/x(x+y)(x-y)
1/(x^2+xy)=(x-y)/x(x+y)(x-y)
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