Question

Find the sum of the first 28 terms of an AP. whose n^th term is given by a_n = 3n − 2.

Answer 1

Given: a_n = 3n − 2

Number of terms, n = 28

∴ a₁ = 3(1) − 2

= 3 − 2

= 1

∴ a₂ = 3(2) − 2

= 6 − 2

= 4

Common difference (d) = a₂ − a₁ 

= 4 − 1

= 3

`S_n = n/2[2a + (n - 1)d]`

`S_(28) = 28/2[2(1) + (28 - 1)3]`

= 14[2 + (27)3]

= 14[2 + 81]

= 14 × 83

= 1162