Ответ:

Объяснение:

Дано:

а₁ = 17, d = -2

Найти: а₈ - ? S₁₂ - ?

а₈ = а₁ + 7d = 17 + 7 * (-2) = 17 - 14 = 3

а₁₂ = а₁ + 11d = 17 + 11 * (-2) = 17 - 22 = - 5

Sn = (а₁ + an)/2)) * n

S₁₂ = (17 - 5)/2)) * 12 = 72

Відповідь: а₈ = 3, S₁₂= 72

[tex]a_{1} = 17 \\ d = - 2 \\ a_{n} = a_{1} + (n - 1)d \\ a_{8} = a_{1} + 7d = 17 + 7 \times ( - 2) = 17 - 14 = 3 \\ a_{12} =a _{1} + 11d = 17 + 11 \times ( - 2) = 17 - 22 = - 5 \\ S_{n} = \frac{a_{1} + a_{n}}{2} n \\ S_{12} = \frac{a_{1} + a_{12}}{2} \times 12 = 6(a_{1} + a_{12}) = \\ = 6 \times (17 - 5) = 6 \times 12 = 72[/tex]