Question

calcular a soma dos N primeiros termos da p.A.(2,10,18,26...)

Answer 1

2 × Sn = (a1 + an) × n

an = a1 + (n-1) × r
an = 2 + (n-1) × 8
an = 2+ 8n -8
an = 8n -6

2 × Sn = ( 2 + 8n -6) × n
2 × Sn = -4n + 8n^2
Sn = -2n + 4n^2