-7x² - 2x + 1 = 0 ⇔ 7x² + 2x - 1 = 0
D= 4 + 4*7 = 32, √D = √32 = 4√2
x₁ = (-2 + 4√2) / 14 = (-1 + 2√2) / 7
x₂ = (-2 -4√2) / 14 = (-1 - 2√2) / 7
x₁ * x₂ = (-1 + 2√2) * (-1 - 2√2) / 49 = - 7 / 49 = - 1/7
-7x² - 2x + 1 = 0 ⇔ 7x² + 2x - 1 = 0
D= 4 + 4*7 = 32, √D = √32 = 4√2
x₁ = (-2 + 4√2) / 14 = (-1 + 2√2) / 7
x₂ = (-2 -4√2) / 14 = (-1 - 2√2) / 7
x₁ * x₂ = (-1 + 2√2) * (-1 - 2√2) / 49 = - 7 / 49 = - 1/7