SOLUTION: The third term of a geometric progression is nine times the first term. The sum of the first six terms is k times the sum of the first two terms. Find the value of k.

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Question 1202408: The third term of a geometric progression is nine times the first term. The sum of the first six terms is k times the sum of the first two terms. Find the value of k.
Answer by ikleyn(52925) About Me  (Show Source):
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The third term of a geometric progression is nine times the first term.
The sum of the first six terms is k times the sum of the first two terms. Find the value of k.
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S%5B6%5D = a%5B1%5D + a%5B2%5D + a%5B3%5D + a%5B4%5D + a%5B5%5D + a%5B6%5D.


Group the terms

S%5B6%5D = (a%5B1%5D+%2B+a%5B2%5D) + (a%5B3%5D+%2B+a%5B4%5D) + (a%5B5%5D+%2B+a%5B6%5D).



You are given  a%5B3%5D = 9%2Aa%5B1%5D.

It implies  a%5B4%5D = 9%2Aa%5B2%5D;  a%5B5%5D = 81%2Aa%5B1%5D;  a%5B6%5D = 81%2Aa%5B2%5D.


Therefore

S%5B6%5D = %28a%5B1%5D+%2B+a%5B2%5D%29 + 9%2A%28a%5B1%5D+%2B+a%5B2%5D%29 + 81%2A%28a%5B1%5D+%2B+a%5B2%5D%29 = %281+%2B+9+%2B+81%29%2A%28a%5B1%5D%2Ba%5B2%5D%29 = 91%2A%28a%5B1%5D%2Ba%5B2%5D%29.


Thus the coefficient k is equal to 91.


ANSWER.  k = 91.

Solved.

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On geometric progressions,  see introductory lessons
    - Geometric progressions
    - The proofs of the formulas for geometric progressions
    - Problems on geometric progressions
    - Word problems on geometric progressions
in this site.

Learn the subject from there.