Ta có:
$\tan\delta = \dfrac{1}{3}$
$\Leftrightarrow \dfrac{\sin\delta}{\cos\delta} = \dfrac{1}{3}$
$\Leftrightarrow \sin\delta = \dfrac{1}{3}\cos\delta$
Ta lại có:
$\sin^2\delta + \cos^2\delta = 1$
$\Rightarrow \left(\dfrac{1}{3}\cos\delta\right)^2 + \cos^2\delta = 1$
$\Leftrightarrow \dfrac{10}{9}\cos^2\delta = 1$
$\Leftrightarrow \cos\delta = \dfrac{3}{\sqrt{10}}$
$\Rightarrow \sin\delta = \dfrac{1}{\sqrt{10}}$
Ta cũng được:
$\cot\delta = \dfrac{1}{\tan\delta} = 3$
