Let $x, y \in \mathbb{R}$ each be a D993: Real number such that
| (i) | \begin{equation} \sqrt{x^2 + y^2} = 1 \end{equation} |
Then
| (1) | \begin{equation} x^2 + x y \leq 1 \end{equation} |
| (2) | \begin{equation} x^2 + x y = 1 \quad \iff \quad x = 1, \; y = 0 \end{equation} |
| (3) | \begin{equation} x^2 + x y \geq 0 \end{equation} |
| (4) | \begin{equation} x^2 + x y = 0 \quad \iff \quad x = y = 0 \end{equation} |