Have given
one interesting question from progressions. This question is from
challenge quiz
I on our website. This is not that difficult, but as with all good
questions, there is a formula-based method of solving this and there is a more
straightforward intuitive way of solving them.
Question
Sum of first 12 terms of a GP is equal to the sum of the first 14 terms in the same GP. Sum of the first 17 terms is 92, what is the third term in the GP?
- 92
- -92
- 46
- 231
Correct Answer : 92. Correct Choice : (1)
Explanatory Answer
Sum of first 12 terms is equal to sum of first 14 terms.
Sum of first 14 terms = Sum of first 12 terms + 13th term + 14th term
=> 13th term + 14th term = 0
Let us assume 13th term = k, common ratio = r. 14th term will be kr.
k + kr = 0
k (1 + r) = 0
=> r = -1 as k cannot be zero
Common ratio = -1.
Now, if the first term of this GP is a, second term would be -a, third would be a and so on
The GP would be a, -a, a, -a, a, -a,...
Sum to even number of terms = 0
Sum to odd number of terms = a
Sum to 19 terms is 92 => a = 92
Third term = a = 92
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