Question: [![enter image description here](https://i.stack.imgur.com/of9bA.png)](https://i.stack.imgur.com/of9bA.png)

How to shade a right triangle like in the picture?

Here's my code:

```
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{tkz-euclide}
\usetikzlibrary{intersections}
\usepackage{amsmath}
\usepackage{siunitx}
\sisetup{output-decimal-marker={,}}

\begin{document}

\begin{tikzpicture}[scale=2]
\coordinate [label=left:$\textcolor{white}A$] (A) at (1cm,-3cm);
\coordinate [label=below:$\textcolor{white}B$] (B) at (1cm,1cm);
\coordinate [label=below:$\textcolor{white}C$] (C) at (-4cm,1cm);
\node at (-3.4cm, .85cm) {$\textcolor{blue}{38^\circ}$};
\tkzMarkAngle[size=0.4cm,color=black,mark=](A,C,B)
\tkzMarkRightAngle[draw=red,size=.2](C,B,A);
\draw [thick=black] (A) -- node[right] {$\textrm{\textcolor{blue}{11}}$} (B) -- node[sloped,above] {$\textrm{\textcolor{blue}{x}}$} (C) -- node[sloped, above] {$\textrm{\textcolor{white}{hypotenuse = 82}}$} (A);
\end{tikzpicture}

\end{document}

```

Don't mind the text's in white; I use it when it's needed.

Answer: 
Another option. Only with Ti*k*Z and its library `angles`.

Then there is something wrong with the numbers, so I'm letting Ti*k*Z do the math for me.

This is the code:

```
\documentclass[border=6mm]{standalone}
\usepackage    {siunitx}
\usepackage    {tikz}
\usetikzlibrary{angles}
\sisetup       {output-decimal-marker={,}}

\begin{document}
\begin{tikzpicture}[scale=0.5,line join=round]
\def\yb{11}
\def\ac{38} % angle C
\pgfmathsetmacro\x{\yb/tan(\ac)}
\pgfmathsetmacro\h{\yb/sin(\ac)} % hypothenuse
\coordinate (A) at (0,0);
\coordinate (B) at (0,-\yb);
\coordinate (C) at (-\x,0);
\draw[thick,fill=blue!20] (A) -- node[right]        {\color{blue}$\yb$}
                          (B) -- node[sloped,above] {\color{white}hypotenuse = \num{\h}}
                          (C) -- node[sloped,above] {\color{blue}$x$} cycle;
% angles, with angles library
\draw[thick] pic [draw] {angle=B--C--A} node [below,xshift=1cm] at (C) {\ang{\ac}};
\draw[thick] pic [draw,angle radius=3mm] {right angle=B--A--C};
\end{tikzpicture}
\end{document}

```

An the output:
[![enter image description here](https://i.stack.imgur.com/25VQB.png)](https://i.stack.imgur.com/25VQB.png)
Something like this? I am assuming that you know how to add the values of the angles.

```
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{tkz-euclide}
\usetikzlibrary{intersections}
\usepackage{amsmath}
\usepackage{siunitx}
\sisetup{output-decimal-marker={,}}

\begin{document}

\begin{tikzpicture}[scale=2]
\coordinate [label=left:$\textcolor{white}A$] (A) at (1cm,-3cm);
\coordinate [label=below:$\textcolor{white}B$] (B) at (1cm,1cm);
\coordinate [label=below:$\textcolor{white}C$] (C) at (-4cm,1cm);
\node at (-3.4cm, .85cm) {$\textcolor{blue}{38^\circ}$};
\tkzMarkAngle[size=0.4cm,color=black,mark=](A,C,B)
\tkzMarkRightAngle[draw=red,size=.2](C,B,A);
\draw [thick=black, fill=white!80!blue] (A) -- node[right] {$\textrm{\textcolor{blue}{11}}$} (B) -- node[sloped,above] {$\textrm{\textcolor{blue}{x}}$} (C) -- node[sloped, above] {$\textrm{\textcolor{white}{hypotenuse = 82}}$} (A);
\end{tikzpicture}

\end{document}

```

[![enter image description here](https://i.stack.imgur.com/me5T7.png)](https://i.stack.imgur.com/me5T7.png)