B₆ = b₁ + 5d
b₁₀ = b₁ + 9d
Система:
b₁ + 5d = 20
b₁ + 9d = 18
b₁ + 5d = 20
-4d = 2 (вычла из 1-го уравнения 2-е)
b₁ = 20 - 5d
d = -1/2
d = -1/2
b₁ = 22.5
b₂₀ = b₁ + 19d = 22.5 - 9.5 = 13
Sn = (b₁ + bn)/2 * n
S₂₀ = (b₁ + b₂₀)/2 * 20 = (22.5+13) * 10 = 35.5 * 10 = 355