[TOÁN 7] So sánh !

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[TEX]\frac{1}{3}A - A = (\frac{1}{3^2} + \frac{1}{3^3} + \frac{1}{3^4} + ... + \frac{1}{3^{101}}) - (\frac{1}{3} + \frac{1}{3^2} + \frac{1}{3^3} + ... + \frac{1}{3^{100}})[/TEX]
$= \frac{- 2}{3} A = \frac{1}{3^{101}} - \frac{1}{3}$
\Rightarrow $A = \frac{1}{3^{101}} - \frac{1}{3} : \frac{- 2}{3}$

\Leftrightarrow $A = \frac{1}{3^{101}} - (\frac{-1}{2})$

\Leftrightarrow $A = \frac{1}{3^{101}} + \frac{1}{2} > \frac{1}{2}$
 
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