SOLUTION: If the 4th term of a geometric sequence is 15 and the 7th term is 405, find the common ratio, and the first term

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Question 1007932: If the 4th term of a geometric sequence is 15 and the 7th term is 405, find the common ratio, and the first term
Answer by CubeyThePenguin(3113) About Me  (Show Source):
You can put this solution on YOUR website!
a = first term, r = third term

ar^3 = 15
ar^7 = 405

Divide equation 2 by equation 1:

(ar^7)/(ar^3) = 405/5 = 81 = r^4

r = 3 or -3, so a = 15/27 = 5/9.