Для функции Лагранжа L(x, y, λ) = x<sup>2</sup> + y<sup>2</sup> + λ(x<sup>2</sup> + y<sup>2</sup> – 2), второй дифференциал d<sup>2</sup>L(x, λ) равен: d<sup>2</sup>L(x, λ) = 2dx<sub>1</sub><sup>2</sup> + 2dx<sub>2</sub><sup>2</sup>, d<sup>2</sup>L(x, λ) = 2dx<sub>1</sub><sup>2</sup> – 2dx<sub>2</sub><sup>2</sup>, d<sup>2</sup>L(x, λ) = dx<sub>1</sub><sup>2</sup> + dx<sub>2</sub><sup>2</sup>, d<sup>2</sup>L(x, λ) = dx<sub>1</sub><sup>2</sup>