If x, y ∈ λ² - Sλ + P with discriminant D = x - y and α = (S² + D²)/(S² - D²) = (x² + y²)/2xy then the given relation implies 2(α + 1/α) = 4 ⇒ α = 1 ⇒ D = x - y = 0 and hence expression equals (7 + 5)/(2 + 1) = 4.