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

Here,

first term, a =

common difference, d

last term, a_n = 4 =

using the nth term formula

a_n = 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