Геометрическая прогрессия bₙ₊₁ = bₙ*q
b₁ = -1/27; q=3.
b₂ = -1/27 * 3 = -1/9
b₃ = -1/9 * 3 = -1/3
b₄ = -1/3 * 3 = -1
b₅ = -1 * 3 = -3
