2187=3^7,能成为等比数列
3^1,3^2,3^3,3^4,3^5,3^6,3^7
S(7)=3+3^2+…+3^7=
=3(1+3+3^2+…+3^6)=
=3(3^7-1)/(3-1)=
=3*(2186/2)=
=3279