设s=1/2+1/6+1/18+1/54+1/162+1/486
3s=3/2+1/2+1/6+1/18+1/54+1/162
3s-s=1-1/486
2s=485/486
s=485/972
1/2+1/6+1/18+1/54+1/162+1/486=485/972