1/(x-1)+1/x+1/(x+1)=13/12
x(x+1)+(x+1)(x-1)+x(x-1)=x(x-1)(x+1)13/12
36*x^2-12=13*x^3-13x
13x(x^2-3x-1)=-12
因为x>1
所以x^2-3x-1