Question

A=7+7^2+7^3+7^4+7^5+7^6+7^7+7^8+7^9+7^11

Answer 1

\(7A=7^2+7^3+...+7^{12}\)

\(\Leftrightarrow A=\dfrac{7^{12}-7}{6}\)

Answer 2

\(7A=7^2+7^3+...+7^{10}+7^{12}\\ 7A-A=7^2+7^3+...+7^{10}+7^{12}-7-7^2-...-7^9-7^{11}\\ 6A=7^{12}-7^{11}+7^{10}-7\\ A=\dfrac{7^{12}-7^{11}+7^{10}-7}{6}\)