\(A=3^{-2}.\left[\left(\dfrac{2}{3}\right)^7+\left(\dfrac{2}{3}\right)^7+...+\left(\dfrac{2}{3}\right)^7\right]\) (có 7 số hạng \(\left(\dfrac{2}{3}\right)^7\))
\(=\dfrac{1}{3^2}.7.\dfrac{2^7}{3^7}\)
\(=\dfrac{7.2^7}{3^9}\)
\(=\dfrac{\dfrac{7}{4}.2^9}{3^9}\)
\(=\left(\sqrt[9]{\dfrac{7}{4}}\right)^9.\left(\dfrac{2}{3}\right)^9\)
\(=\left(\sqrt[9]{\dfrac{7}{4}}.\dfrac{2}{3}\right)^9\)
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