令A=1-1/3-1/9-1/27-1/81-1/243-1/729
则:A/3=1/3-1/9-1/27-1/81-1/243-1/729-1/2187
两式相减,得到:2A/3=1-2/3+1/2187=730/2187
所以:A=365/729
令A=1-1/3-1/9-1/27-1/81-1/243-1/729
则:A/3=1/3-1/9-1/27-1/81-1/243-1/729-1/2187
两式相减,得到:2A/3=1-2/3+1/2187=730/2187
所以:A=365/729