b₁ = 1 b₂ = 5 bₙ = 125
b₂ = b₁ * q
q = b₂ : b₁ = 5
bₙ = b₁ * qⁿ⁻¹
125 = 1 * 5ⁿ⁻¹
5ⁿ⁻¹ = 5³
n - 1 = 3
n = 4
ответ : b₄ = 125
