The geometric sequence 20, 60, 180, 540,.........., 393 660 has how many terms?

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Answer 1 · 2015-11-08T10:00:59

$n = 10$

$A_n = A_1r^(n - 1)$

$A_1 = 20$

$A_2 = 60 = A_1r^(2-1)$

$=> 60 = 20r$
$=> r = 3$

$A_n = 393660 = A_1r^(n - 1)$

$=> 393660 = 20*3^(n -1)$

$=> 19683 = 3^(n-1)$

$=> 3^9 = 3^(n - 1)$
$=> 9 = n - 1$

$=> n = 10$