Question

1) Tính \(S=-1+\dfrac{1}{10}-\dfrac{1}{10^2}+...+\dfrac{\left(-1\right)^n}{10^{n-1}}\)

2) Tính \(S=1+\dfrac{1}{3}+\dfrac{1}{3^2}+...+\dfrac{1}{3^{n-1}}\)

Answer 1

1:

\(S=-\left(1-\dfrac{1}{10}+\dfrac{1}{10^2}-...-\dfrac{1}{10^{n-1}}\right)\)

\(=-\left[\left(-\dfrac{1}{10}\right)^0+\left(-\dfrac{1}{10}\right)^1+...+\left(-\dfrac{1}{10}\right)^{n-1}\right]\)

\(u_1=\left(-\dfrac{1}{10}\right)^0;q=-\dfrac{1}{10}\)

\(\left(-\dfrac{1}{10}\right)^0+\left(-\dfrac{1}{10}\right)^1+...+\left(-\dfrac{1}{10}\right)^{n-1}\)

\(=\dfrac{\left(-\dfrac{1}{10}\right)^0\left(1-\left(-\dfrac{1}{10}\right)^{n-1}\right)}{-\dfrac{1}{10}-1}\)

\(=\dfrac{1-\left(-\dfrac{1}{10}\right)^{n-1}}{-\dfrac{11}{10}}\)

=>\(S=\dfrac{1-\left(-\dfrac{1}{10}\right)^{n-1}}{\dfrac{11}{10}}\)

2:

\(S=\left(\dfrac{1}{3}\right)^0+\left(\dfrac{1}{3}\right)^1+...+\left(\dfrac{1}{3}\right)^{n-1}\)

\(u_1=1;q=\dfrac{1}{3}\)

\(S_{n-1}=\dfrac{1\cdot\left(1-\left(\dfrac{1}{3}\right)^{n-1}\right)}{1-\dfrac{1}{3}}\)

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

Answer 2

\(1,\) Ta có \(\left\{{}\begin{matrix}q=\dfrac{u_2}{u_1}=\dfrac{1}{10}:\left(-1\right)=-\dfrac{1}{10}\\u_1=-1\end{matrix}\right.\)

Vậy \(S=-1+\dfrac{1}{10}-\dfrac{1}{10^2}+...+\dfrac{\left(-1\right)^n}{10^{n-1}}=\dfrac{-1}{1-\left(-\dfrac{1}{10}\right)}=-\dfrac{10}{11}\)

\(2,\) Ta có \(\left\{{}\begin{matrix}q=\dfrac{u_2}{u_1}=\dfrac{1}{3}\\u_1=1\end{matrix}\right.\)

Vậy \(S=1+\dfrac{1}{3}+\dfrac{1}{3^2}+...+\dfrac{1}{3^{n-1}}=\dfrac{1}{1-\dfrac{1}{3}}=\dfrac{3}{2}\)