In a geometric sequence, #a_n=-1215#, #a_1= 5# and #r=- 3#. What is the value of n?

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Answer 1 · 2017-02-02T11:36:15

The value of #n# is #6#.

The formula is #a_n = a_1 * r^(n - 1)#

#-1215 = 5 * (-3)^(n - 1)#

Divide by five both sides
#-243 = (-3)^(n-1)#

Signal problem for logarithm
Try and check
#243 = 3^(n - 1)#

#log_3 243 = n - 1 = 5#

#n = 6#

Checking
#-243 = (-3)^5#
OK