Question

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

Answer 1

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