вычислите значение суммы 6,8+9,3+11,8+...+56,8

a₁= 6.8a₂ = 9.3d = 9.3 - 6.8 = 2.5 an = 56.8Sn - ?an = a₁ + d (n - 1) = 56.86.8 + 2.5 (n - 1) = 56.86.8 + 2.5n - 2.5 = 56.82.5n = 52,5n = 21S₂₁ = (a₁ + a₂₁)/2*  21 = 10,5* (6,8 + 56,8) = 667,8