125, - 100, 80...
b₁ = 125, b₂ = - 100
q = b₂ / b₁ = - 100 / 125 = - 4/5
b₅ = b₁ · q⁴
b₅ = 125 · (- 4/5)⁴ = 5³ · 4⁴ / 5⁴ = 4⁴ / 5 = 256/5 = 51,2
