12. -x² - 2x + 15 = 0

a = -1, b = -2, c = 15 D = (-2)² - 4 * (-1) * 15 = 4 + 60 = 64 x₁ = (2 + √64) / 2 * (-1) = (2 + 8) / -2 = 10 / -2 = -5 x₂ = (2 - √64) / 2 * (-1) = (2 - 8) / -2 = -6 / -2 = 3 Ответ: x₁ = -5, x₂ = 3