find sn and s20

Question

If t _n= 2n +1, find S_n and evaluate S ₂₀.

Answer 1

If t_n = 2n + 1; n = 1, t₁ = 3 and d = 2; S_n = n(t₁ + t_n)/2 = n(3 + 2n + 1)/2 = n(n + 2)

n = 20; S₂₀ = 20(20 + 2) = 440