[tex]\cos a=\frac{-13}{15} \ |^2\\ \\ \Rightarrow \cos^2 a=\frac{13^2}{15^2}\\ \\ \sin^2a+\cos^2 a=1\\ \\ \Rightarrow \sin ^2a=1-\frac{13^2}{15^2}\\ \\ \Rightarrow \sin^2a=\frac{225-169}{225}\\ \\ \sin^2a=\frac{56}{225}\\ \\ \Rightarrow \sin a=-\sqrt{\frac{56}{225}} \ (in \ cadranul \ 3, \ \sin x<0)\\ \\ \sin a=0.4988 \ (aprox.)[/tex]
