Question

How do you find S20 for the geometric series 4 + 12 + 36 + 108 + …?

Answer 1

$sn = \frac{n}{2}(2a + (n - 1)d)$

$a = 4 \: \: \: \: d = 12 - 4 = 8 \: \: \: \: \: n = 20$

$s20 = \frac{20}{2} (2(4) + (20 - 1)(8) \\ = 10(8 + (19)8) \\ 10(160) \\ = 1600$

So your answer is 1600