Question

A=1+3+3^2+3^3+3^4+3^5+3^6+3^7  tinh 2A

Answer 1

\(A=1+3+3^2+3^3+3^4+3^5+3^6+3^7\)

\(\Rightarrow3A=3+3^2+3^3+3^4+3^5+3^6+3^7+3^8\)

\(\Rightarrow2A=3A-A=3+3^2+3^3+3^4+3^5+3^6+3^7+3^8-1-3-3^2-3^3-3^4-3^5-3^6-3^7\)

\(\Rightarrow2A=3^8-1\)

Answer 2

\(3A=3+3^2+3^3+3^4+3^5+3^6+3^7+3^8\\ 3A-A=2A=3^8-1\)