3、原式=x/(x-1)-1/(x+1)=x(x+1)(x-1)-(x-1)/(x+1)(x-1)=1/(x+1)(x-1)
4、原式=(a-2)/(a^2-2a+1)-1/(a-1)=(a-2)/(a-1)^2-1/(a-1)=(a-2-a+1)/(a-1)^2=-1/(a-1)^2
5、原式=a^2/(a-3)-a-3=a^2/(a-3)-(a+3)=(a^2-a^2+9)/(a-3)=9/(a-3)
6、原式=x/(-x^2+1)*(x^2+x)/x^2=-x/(x+1)(x-1)*x(x+1)/x^2=-1/(x+1)
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