5. x² - x - 20 = 0

a = 1, b = -1, c = -20 D = (-1)² - 4 * 1 * (-20) = 1 + 80 = 81 x₁ = (1 + √81) / 2 * 1 = (1 + 9) / 2 = 10 / 2 = 5 x₂ = (1 - √81) / 2 * 1 = (1 - 9) / 2 = -8 / 2 = -4 Ответ: x₁ = 5, x₂ = -4