a₁ + 2d = 14
a₁ + 9d = 29
29 - 14 = 15 = 7d
d = 15/7
a₁ = 14 - 30/7 = (98 - 30)/7 = 68/7
S₂₀ = (2a₁ + 19d)/2 * 20 = 10(136/7 + 19*15/7) = 4210/7
ответ: 601 3/7
по формуле an = a1 + d(n - 1)
29= а1+9d
- {
14 =a1 +2d
15=7d
d=15/7
а1=29- 9⋅ 15/7=135/7
По формуле
Sn = (2a1 + d(n - 1)) ⋅ n / 2
S25 = (2⋅135/7 +15/7⋅24)⋅25/2=1125
a₁ + 2d = 14
a₁ + 9d = 29
29 - 14 = 15 = 7d
d = 15/7
a₁ = 14 - 30/7 = (98 - 30)/7 = 68/7
S₂₀ = (2a₁ + 19d)/2 * 20 = 10(136/7 + 19*15/7) = 4210/7
ответ: 601 3/7
по формуле an = a1 + d(n - 1)
29= а1+9d
- {
14 =a1 +2d
15=7d
d=15/7
а1=29- 9⋅ 15/7=135/7
По формуле
Sn = (2a1 + d(n - 1)) ⋅ n / 2
S25 = (2⋅135/7 +15/7⋅24)⋅25/2=1125