Question

Tinh gia tri bieu thuc

D = -1/3 +1/3²-1/3³+..........+1/3⁹⁸-1/3⁹⁹

Answer 1

\(D=\left(-\dfrac{1}{3}\right)^1+\left(-\dfrac{1}{3}\right)^2+...+\left(-\dfrac{1}{3}\right)^{98}+\left(\dfrac{-1}{3}\right)^{99}\)

\(\Leftrightarrow\left(-\dfrac{1}{3}D\right)=\left(-\dfrac{1}{3}\right)^2+...+\left(-\dfrac{1}{3}\right)^{99}+\left(-\dfrac{1}{3}\right)^{100}\)

\(\Leftrightarrow D\cdot\dfrac{-4}{3}=\dfrac{1^{100}}{3^{100}}-\left(-\dfrac{1}{3}\right)=\dfrac{1}{3^{100}}+\dfrac{1}{3}=\dfrac{1+3^{99}}{3^{100}}\)

\(\Leftrightarrow D=\dfrac{3^{99}+1}{3^{100}}:\dfrac{-4}{3}=\dfrac{3^{99}+1}{-4\cdot3^{99}}\)