Все , прогрессия с 11 по 18 умоляю

11) a₁₂ = 75     d = 8

a₁₂ = a₁ + 11d

a₁ = a₁₂ - 11d = 75 - 11 * 8 = 75 - 88 = - 13

12) a₁₄ = - 63       a₁ = 28

a₁₄ = a₁ + 13d

13d = a₁₄ - a₁ = - 63 - 28 = - 91

d = - 91 : 13 = - 7

13) a₁ = - 55      aₙ = 25      d = 5

aₙ = a₁ + d(n - 1)

25 = - 55 + 5(n - 1)

25 = - 55 + 5n - 5

5n = 25 + 60

5n = 85

n = 1 7

14) a₁₈ = - 21      d = 4

a₁₈ = a₁ + 17d

a₁ = a₁₈ - 17d = - 21 - 17 * 4 = - 21 - 68 = - 89

a₂₉ = a₁ + 28d = - 89 + 28 * 4 = - 89 + 112 = 23

15) a₂₅ = - 51     d = - 6

a₂₅ = a₁ + 24d

a₁ = a₂₅ - 24d = - 51 - 24 * (-6) = - 51 + 144 = 93

a₁₆ = a₁ + 15d = 93 + 15 * (- 6) = 93 - 90 = 3

17)a₅ = 34       aₙ= - 74      d = - 9  

a₅ = a₁ + 4d

a₁ = a₅ - 4d = 34 - 4 * (- 9) = 34 + 36 = 70

aₙ = a₁ + d(n - 1)

- 74 = 70 - 9(n - 1)

- 144 = - 9n + 9

9n = 153

n = 17

18) aₙ = - 59       a₃₀ = 13     d = 6

a₃₀ = a₁ + 29d

13 = a₁ + 29 * 6

a₁ = 13 - 174 = - 161

aₙ = a₁ + d(n - 1)

- 59 = - 161 + 6(n - 1)

102 = 6n - 6

6n = 108

n = 1 8