Question

Помогите!!! Даю 55 баллов
Решить третий номер,
а б в

Answer 1

$b1 = 12 \\ d = - 3 \\ \\ 1)bn = - 6 \\ bn = b1 + d(n - 1) \\ - 6= 12 - 3(n - 1) \\ 15 - 3n = - 6 \\ - 3n = - 21 \\ n = 7 \\ \\ 2)bn = 0 \\ 15 - 3n = 0 \\ - 3n = - 15 \\ n = 5 \\ \\ 3)bn = 9 \\ 15 - 3n = 9 \\ - 3n = - 6 \\ n = 2$