Question

an = 2n + 1. Найти S17

Answer 1

а₁=2*1+1=3
а₂=2*2+1=5
а₃=2*3+1=7
а₁₇=2*17+1=35
S=(а₁+а₁₇)*17/2=(3+35)*17/2=19*17=323