Week 2 – geometric sequences

Posted on February 13, 2018 by pouya

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4,20,100,500,2500,…

$S_{10}=\frac{4(5^{10}-1)}{5-1}=9765624$

Its a geometric sequence and i calculated the sum of first 10 terms with this formula:

$S_n=\frac{a_n(a^n-1)}{a-1}$