Definition. Let $\ell_1$, $\ell_2$ be two intersecting lines defined by unit normals $n_1$, $n_2$. The exterior angle $\hat\alpha$ of these two lines is given by \[\cos\hat\alpha=\langle n_1,n_2\rangle\,.\] The interior angle $\alpha=\pi-\hat\alpha$ is given by \[\cos\alpha=-\langle n_1,n_2\rangle\,.\] In particular, two lines … Continue reading Lecture 20