`~rai~`
\(c)\text{Ta có:}\sin^2\alpha+\cos^2\alpha=1\\\Leftrightarrow (0,8)^2+\cos^2\alpha=1\\\Leftrightarrow \cos^2\alpha=0,36\\\Leftrightarrow \cos\alpha=0,6\quad\text{(vì }\cos\alpha>0)\\\Rightarrow \cot\alpha=\dfrac{\cos\alpha}{\sin\alpha}=\dfrac{0,6}{0,8}=\dfrac{3}{4}.\\A=\tan^{2012}\alpha.\cot^{2013}\alpha\\\quad=(\tan^{2012}\alpha.\cot^{2012}\alpha).\cot\alpha\\\quad=1.\cot\alpha\quad\text{(vì }\tan\alpha.\cot\alpha=1)\\\quad=1.\dfrac{3}{4}=\dfrac{3}{4}.\\\text{Vậy }\cos\alpha=0,6\quad\text{và A=}\dfrac{3}{4}.\)
