Ответы на вопрос:
A₁= 6.8 a₂ = 9.3 d = 9.3 - 6.8 = 2.5 an = 56.8 sn - ? an = a₁ + d (n - 1) = 56.8 6.8 + 2.5 (n - 1) = 56.8 6.8 + 2.5n - 2.5 = 56.8 2.5n = 52,5 n = 21 s₂₁ = (a₁ + a₂₁)/2* 21 = 10,5* (6,8 + 56,8) = 667,8
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