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#