Решите плз!!!!!!!!!!!!!!!!!!!!!!!!. Created by valenok444. algebra-ru

Решите плз!!!!!!!!!!!!!!!!!!!!!!!!...

October 2021 1 5 Report

Решите плз!!!!!!!!!!!!!!!!!!!!!!!!

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nKrynka Решение
1) Всего чисел
91 – 30 + 1 = 62
n = 62
a₁ = 30
a₆₂ = 91
S₆₂ = [(30 + 91)*62] / 2 = 121 * 31 = 3751
2) Допустим, что существует арифметическая прогрессия (аn),
где a₃ = - 4; a₆ = - 13; a₁₀ = - 25, тогда
an = a₁ + d(n – 1)
a₃ = a₁ + d*2 = - 4
a₆ = a₁ + d*5 = - 13
Решим систему:
a₁ + d*2 = - 4
a₁ + d*5 = - 13

3d = - 9
d = - 3
a₁ = - 4 – 2d = - 4 + 6 = 2
Если a₁ = 2 ; d = - 3 , то a₁₀ = a1 + d*9 = 2 + 9*(-3) = - 25
Да существует с a₁ = 2; d = - 3
an = a₁ + d(n – 1)
an = 2 – 3*(n – 1)
3) an = 2n - 4
a₁ = 2*1 - 4 = - 2
a₂ = 2*2 – 4 = 0
d = 0 – (-2) = 2
a₃ = 2*3 – 4 = 2
a₂₀ = 2*20 – 4 = 36
a₃₀ = 2*30 – 4 = 56
Sn = [(a1 + an)* n]/ 2
Всего чисел 30 – 20 +1=11
S₁₁ = [(36 + 56)*11] / 2 = 506

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