x² + px + q = 0

x1 + x2 = -p

x1*x2 = q

обратная теорема Виета

x² + 13x - 15 = 0

x1 + x2 = -13

x1*x2 = -15

(x₁² + x₂²)/4x₁*x₂ = ((x₁ + x₂)² - 2x₁x₂)/4x₁x₂ = ((-13)² - 2*(-15))/4*(-15) = (169 + 30)/(--60) = - 199/60