Question

Considere a P. A. (5, 4, 3, 2,...). Calcule a soma dos 20 primeiros termos

Answer 1

P.A.(5,4,3,2,....)
R = A 2 - A 1
R = 4 - 5
R = -1
a 20 = A 1 + (N - 1 ).R
A 20 = 5 + (20 - 1 ).-1
A 20 = 5 + (19).-1
A 20 = 5 - 19
A 20 = -14
S 20 = 20.(a 1 + a 20 ) / 2
S 20 = 20 (5 -14) / 2
S 20 = 20 .(-9) / 2
S 20 = -180 / 2
S 20 = -90