Q19 of 89 Page 51

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

Here,


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


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


= 2a + 17d





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