Ta có: \(A=\frac53+\frac{5}{3^2}+\cdots+\frac{5}{3^{20}}\)
=>\(3A=5+\frac53+\cdots+\frac{5}{3^{19}}\)
=>\(3A-A=5+\frac53+\cdots+\frac{5}{3^{19}}-\frac53-\frac{5}{3^2}-\cdots-\frac{5}{3^{20}}\)
=>\(2A=5-\frac{5}{3^{20}}=\frac{5\cdot3^{20}-5}{3^{20}}\)
=>\(A=\frac{5\cdot3^{20}-5}{2\cdot3^{20}}\)