Question

1) tính \(S=1+\dfrac{1}{3}+\dfrac{1}{9}+...+\dfrac{1}{19683}\)

2) tính \(S=1+\dfrac{1}{5}+\dfrac{1}{25}+...+\dfrac{1}{78125}\)

Answer 1

1: \(S=1+\dfrac{1}{3}+\dfrac{1}{9}+...+\dfrac{1}{3^9}\)

\(=\left(\dfrac{1}{3}\right)^0+\left(\dfrac{1}{3}\right)^1+...+\left(\dfrac{1}{3}\right)^9\)

u1=1; q=1/3

\(S_9=\dfrac{u1\cdot\left(1-q^9\right)}{1-q}=\dfrac{1\left(1-\left(\dfrac{1}{3}\right)^9\right)}{1-\dfrac{1}{3}}\)

\(=\dfrac{3}{2}\left(1-\dfrac{1}{3^9}\right)\)

2:

\(S=\left(\dfrac{1}{5}\right)^0+\left(\dfrac{1}{5}\right)^1+...+\left(\dfrac{1}{5}\right)^7\)

\(u1=1;q=\dfrac{1}{5}\)

\(S_7=\dfrac{1\cdot\left(1-q^7\right)}{1-q}=\dfrac{1-\left(\dfrac{1}{5}\right)^7}{1-\dfrac{1}{5}}=\dfrac{5}{4}\left(1-\dfrac{1}{5^7}\right)\)