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Asked By:  Sanjayraj Balasara

The 5th and 15th terms of an AP are 13 and -17 respectively. Find the sum of the first 21 terms of the AP.

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Answer By: Excellant

Let us assume 5th term as 1st term, then 15th term will be 11th term. Difference between them is 13 - (-17) = 30. So, common difference is -3.Now, Difference between 21 and 15 is 6 so, 21st term = -17 - 3 x 6 = -17 - 18 = -35

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