x₁ + x₂ = -p
x₁*x₂ = q
x² - 9x + 15 = 0
x₁ + x₂ = 9
x₁*x₂ = 15
(x₁)² + (x₂)² = (x₁)² + (x₂)² + 2x₁x₂ - 2x₁x₂ = (x₁ + x₂)² - 2x₁x₂ = 9² - 2*15 = 81 - 30 = 51
x₁ + x₂ = -p
x₁*x₂ = q
x² - 9x + 15 = 0
x₁ + x₂ = 9
x₁*x₂ = 15
(x₁)² + (x₂)² = (x₁)² + (x₂)² + 2x₁x₂ - 2x₁x₂ = (x₁ + x₂)² - 2x₁x₂ = 9² - 2*15 = 81 - 30 = 51