Question

calcule a soma dos quinze primeiros termos da P.A. (-45, -41, -37, -33,...).

Answer 1

an = A1 + ( n - 1 ) × r
an = - 45 + ( 15 - 1 ) × 4
an = - 45 + 14× 4
an = - 45 + 56
an = 11

Sn = ( A1 + An ) × n / 2
Sn = ( -45 + 11 ) × 15 / 2
Sn = - 34 × 15/ 2
Sn = - 17 × 15
Sn = - 255