1/
$\cot a=\dfrac{1}{\tan a}=\dfrac{1}{2}$
$\dfrac{1}{\cos^2a}=1+\tan^2a$
$\Rightarrow \cos a=\dfrac{\sqrt5}{5}$
$\cos a=\sqrt{1-\sin^2a}=\dfrac{2\sqrt5}{5}$
2/
$\cos a=\sqrt{1-\sin^2a}=\dfrac{1}{2}$
$\tan a=\dfrac{\sin a}{\cos a}=\sqrt 3$
$\cot a=\dfrac{1}{\sqrt 3}$