a(n) = a(n - 1) + d
d = a(2) - a(1)
a(1) = 74
a(2) = 68
d = 68 - 74 = -6
a(3) = a(2) + d = 68 - 6 = 62
арифметическая прогрессия 74, 68, 62, 56, 50
