Q17 of 38 Page 5

Find the sum of the two middle most terms of an AP

Given: AP


To find:


Formula Used: nth term of an AP is:


an = a + (n - 1) d


Explanation:


first term, a =


common difference, d


last term, an = 4 =


using the nth term formula


an = a + (n - 1) d



13 = - 4 + (n - 1) [ multiplication by 3 on the both side]


13 + 4 = n - 1


n = 18


as n is even middle terms will be i.e. 9th and 10th terms


a9 + a10 = a + 8d + a + 9d


= 2a + 17d





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