Así, $$\mathop {\lim }\limits_{\theta \to \pi } {{sen\theta } \over {tg\theta }} = \mathop {\lim }\limits_{\theta \to \pi } {{sen\theta } \over {{{sen\theta } \over {\cos \theta }}}} = \mathop {\lim }\limits_{\theta \to \pi } \cos \theta = - 1$$