Question

Найдите тридцатый член артфмет прогрессии-31;-28;-25

Answer 1

$a_1 = -31, a_2 = -28, a_3 = -25;\\ a_n = a_1 + (n - 1)d;\\ a_2 = a_1 + d, -28 = -31 + d, -28 + 31 = d, d = 3\\ a_{30} = -31 + 29cdot 3 = -31 + 87 = boxed{56}$