令s=1/3+1/9+1/27+1/81所以3s=1+1/3+1/9+1/27所以2s=3s-s=1-1/81=80/81所以 s=1/3+1/9+1/27+1/81=40/81
最简便的算法就是通分1/3+1/9+1/27+1/81=(27+9+3+1)/81=41/81
