Đặt A = 1 + 3 + 32 + 33 + .... + 320
=> 3A = 3 + 32 + 33 + .... + 321
=> 3A - A = 321 - 1
=> 2A = 321 - 1
=> A = \(\frac{3^{21}-1}{2}\)
Đặt \(A=1+3+3^2+...+3^{20}\)
\(3A=3+3^2+3^3+...+3^{21}\)
\(\Rightarrow3A-A=3^{21}-1\)
\(\Rightarrow2A=3^{21}-1\Rightarrow A=\left(3^{21}-1\right):2\)
Ủng hộ mk nha!!!
Đặt A=1+3+3^2+...+3^20
3A=3+3^2+...+3^21
3A-A=(3+3^2+...+3^21)-(1+3+...+3^20)
2A=3^21-1
A=(3^21-1)2