What is the slope of the line from the origin to the point $A$ in Figure 4.2? The equation of this line, the $\yy'$-axis, is \begin{equation} x = \yy \tanh\beta \end{equation} Consider now a line with equation \begin{equation} x' = \yy' \tanh\alpha \end{equation} What is its (unprimed) slope? Again, slopes don't add, but (hyperbolic) angles do; the answer is that \begin{equation} x = \yy \tanh(\alpha+\beta) \end{equation} which can be expressed in terms of the slopes $\tanh\alpha$ and $\tanh\beta$ using (7) of §4.1. As discussed in more detail in the next chapter, this is the Einstein addition formula!