Ta có
$\tan a = -\dfrac{15}{7}$, suy ra $cot a = -\dfrac{7}{15}$
Ta có
$\dfrac{1}{\cos^2a} = 1 + \tan^2a$
$<-> \dfrac{1}{\cos^2a} = \dfrac{274}{49}$
$<-> \cos a = \pm \dfrac{7}{\sqrt{274}}$
Do $\dfrac{\pi}{2} < a < \pi$ nên $\sin a < 0$, $\cos a > 0$
Vậy $\cos a = \dfrac{7}{\sqrt{274}}$
Áp dụng cthuc ta có
$\sin a = \sqrt{1 - \cos^2a} = -\dfrac{15}{\sqrt{274}}$
