Question

soma dos 20° termos da P.A (2,5,8,11,14..)

Answer 1

A1 = 2
r = 3
A20 = 2 + (19*3) = 2 + 57 = 59

S20 = ((2 + 59) * 20)/2 = 61 * 10 = 610

Answer 2

P.A.(2,5,8,11,14,17,20,23,26,29,32,35,38,41,44,47,50,53,56,59,....)
r = a 2 - a 1
r = 5 - 2
r = 3
a 20 = a 1 + (n - 1).r
a 20 = 2 + (20 - 1 ).3
a 20 = 2 + (19 . 3)
a 20 = 2 + 57
a 20 = 59
S n = n (a 1 +a n ) / 2
S 20 = 20 (2 + 59 ) / 2
S 20 = 20 (61) / 2
S 20 = 10 . 61
S 20 = 610