Question

a2=9 a6=21 d-? a11=? s20-?

Answer 1

Ответ:

Пошаговое объяснение:

a₂ =9

a₆ = 21

a₂ = a₁ +d

a₆ = a₁ +5d

a₆ - a₂ = a₁ +5d -a₁ -d = 21-9 = 12

4d = 12  d = 3

a₁ = 9-3=6

a₁₁ = 6 + 3*10 = 36

S₂₀ = (2a₁+d(n-1))*n/2 = (2*6+3*19)*20/2=(12+57)*10 = 690