[tex]\sin B \cdot \text{tg}\, B = \dfrac{b^2}{ac} \\ \\ \text{In triunghiul dreptunghic: }\sin \alpha = \dfrac{cateta~opusa}{ipotenuza}, \text{tg}\alpha = \dfrac{cateta~opusa}{cateta~alaturata}\\ \\\dfrac{AC}{BC}\cdot \dfrac{AC}{AB} = \dfrac{b^2}{ac} \\ \\\Rightarrow\dfrac{AC^2}{BC\cdot AB}= \dfrac{b^2}{ac}\quad (A) \\ \\ \Rightarrow \triangle ABC \rightarrow \text{triunghi dreptunghic}[/tex]
