When do we have to put Sn =n/2 (a+an)

1.	When do we have to put Sn =n/2 (a+an)
Answer» We put "Sn = n/2 ( a + an )" when we know the value of "an" (last term of an AP) to find the sum of "n" terms when we know the last term of an AP. Sorry I didn\'t know

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