Question

Свойства арифметической прогрессии

Answer 1

1) S = (a1 + an)/2 * n
n = 58 - 21 + 1 = 38

S = (21 + 58)/2 * 38 = 79 * 19 = 1501


2) cn = c1 + d*(n-1),
d = 2
c5 = c1 + d*(n-1) = c1 + 4d = c1 + 8
c1 + 8 = 14
c1 = 6