Cho góc alpha thỏa mãn sin \alpha  = 12/13

ChọnD.    \({\sin ^2}\alpha + {\cos ^2}\alpha = 1 \Rightarrow {\cos ^2}\alpha = 1 - {\sin ^2}\alpha = 1 - {\left( {\frac{{12}}{{13}}} \right)^2} = \frac{{25}}{{169}} \Rightarrow \cos \alpha = \pm \frac{1}{{13}}\) vì \(\frac{\pi }{2} < \alpha < \pi \)nên \(\cos \alpha < 0 \Rightarrow \cos \alpha = - \frac{1}{{13}}\).