Respuestas

2013-04-08T19:25:51+02:00

d = 7

A1 = 21

An = 77

n?

S?

An = A1 + (n - 1) d;   77 = 21 + (n - 1) 7;    77 = 21 + 7n - 7;   77 - 21 + 7 = 7n;   63 = 7n;

n = 63 / 7;   n = 9

 

S = [(A1 + An) / 2].n;    S = [ (21 + 77) / 2] . 9;    S = 441