4. Найдите сумму четырех первых членов геометрической прогрессии (bn), в которой:

а) b₃ = 1/25, b₄ = 1/125. q = b₄ / b₃ = (1/125) / (1/25) = 1/5. b₁ = b₃ / q² = (1/25) / (1/25) = 1. S₄ = 1 * (1 - (1/5)⁴) / (1 - 1/5) = (1 - 1/625) / (4/5) = (624/625) * (5/4) = 156/125 = 1.248.
б) b₂ = 6, b₄ = 24, q > 0. q² = b₄ / b₂ = 24 / 6 = 4. q = 2. b₁ = b₂ / q = 6 / 2 = 3. S₄ = 3 * (1 - 2⁴) / (1 - 2) = 3 * (1 - 16) / (-1) = 3 * (-15) = -45.