This is an example of using the align* and tikz environments to typeset the worked example on how to find an unknown side length in a right-angled triangle using trigonometry.
The code used in the video is (minus less than and greater than signs which aren't allowed in vid description):
\begin{tikzpicture}[scale=3.0,cap=round,=(greater than sign missing here)latex]
\coordinate (A) at (0,0);
\coordinate (B) at (1,0);
\coordinate (C) at (1,-1.73205);
\draw [blue](A) -- (B) node [above, midway] {$\text{opp}=10$};
\draw [blue](B) -- (C) node [right, midway] {$\text{adj}=a$};
\draw [blue](C) -- (A) ;
\pic [draw,angle radius=1cm,blue, -, "$11\degree$", angle eccentricity=1.3] {angle = B--C--A};
\tkzMarkRightAngle[draw=blue,size=.10](A,B,C)
\end{tikzpicture}
\begin{align*}
\intertext{Here I have $\text{opp}=10$ and $\text{adj}=a$ so: \endgraf}
\tan 11\degree &= \dfrac{\text{opp}}{\text{adj}} &&&&&\\[2\jot]
&= \dfrac{10}{a} &&&&&\\[2\jot]
a &= \dfrac{10}{\tan 11\degree} &&&&&\\[2\jot]
&= 51.4455\ldots &&&&&\\
&= 51.45\text{ (to 2 d.p.).}
\end{align*}
