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
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