Question

Find the sum of first 28 terms of an A.P. whose n^th term is 4n – 3

Answer 1

n = 28

t_n = 4n – 3

t₁ = 4 × 1 – 3 = 1

t₂ = 4 × 2 – 3 = 5

t₂₈ = 4 × 28 – 3

= 112 – 3 = 109

∴ a = 1, d = t₂ – t₁ = 5 – 1 = 4

l = 109

S_n = `"n"/2` (2a + (n – 1)d)

S₂₈ = `28/2` (2 × 1 + 27 × 4)

= 14(2 + 108)

= 14 × 110

= 1540