Question

Find the sum of the following

3, 7, 11, … up to 40 terms

Answer 1

3, 7, 11, ... upto 40 terms.

a = 3, d = t₂ – t₁ = 7 – 3 = 4

n = 40

S_n = `"n"/2` (2a + (n – 1)d)

S₄₀ = `20/2` (2 × 3 + 39d)

= 20(6 + 39 × 4)

= 20(6 + 156)

= 20 × 162

= 3240