Let $\sin$ be the D1931: Standard real sine function.
Let $\cos$ be the D1927: Standard real cosine function.
The standard real tangent function is the D4364: Real function
\begin{equation}
\tan : \left( - \frac{\pi}{2}, \frac{\pi}{2} \right) \to \mathbb{R}, \quad
\tan(x) = \frac{\sin(x)}{\cos(x)}
\end{equation}