cái này chả phải tớ đã trả lời rồi sao
\(A=1+3+3^2+3^3+...+2^{2015}\)
\(3A=3.\left(1+3+3^2+3^3+...+3^{2015}\right)\)
\(3A=3+3^2+3^3+3^4+...+3^{2016}\)
\(3A-A=\left(3+3^2+3^3+3^4+...+3^{2016}\right)-\left(1+3+3^2+3^3+...+3^{2015}\right)\)
\(2A=3^{2016}-1\)
\(A=\left(3^{2016}-1\right):2\)