1-1/3-1/9-1/27-1/81-1/243-1/729-1/2187=1-(1/3+1/9+1/27+1/81+1/243+1/729+1/2187)=1-(1/3)*[1-(1/3)^7]/[1-(1/3)]=1-(1/3)*[1-(1/2187)]/(2/3)=1-(1/2)*(2186/2187)=1094/2187