s=原式；
=lim[(1/a+1/b)+(1/a^2+1/b^2)+...+(1/a^n+1/b^n)]
=lim[(1/a+1/a^2+...+1/a^n)+(1/b+1/b^2+...+1/b^n)]
=1/(a-1)+1/(b-1)