Question

The sum of first 13 terms of an A.P.is 21 and the sum of first 21 terms is 13.The sum of first 34 terms​

Answer 1

Answer:

The sum of the first 13 terms is 13a + 78d = 21. ... So the nth sum is 283n/91 - 34n(n-1)/273.

Step-by-step explanation:

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