[Toán 7] Luỹ thừa

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letieungoclinhkt

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iceghost

$A = 1+3+3^2+3^3+...+3^{30}$ (1)
\Rightarrow $3A = 3+3^2+3^3+3^4+...+3^{31}$ (2)
Lấy (2) trừ cho (1) ta được
$2A = 3^{31} - 1$
$2A + 1 = 3^{31}$
 
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