\documentclass[margin=4mm, tikz]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{angles, quotes, babel}
\usepackage{amsmath, amssymb, amsfonts}
\begin{document}
% Gegebene Größen
\pgfmathsetmacro{\Alpha}{77}%
\pgfmathsetmacro{\r}{4/(2*sin(\Alpha))} %
\begin{tikzpicture}[scale=0.7,
font=\footnotesize,
Punkt/.style 2 args={ label={[#1]:$#2$} },
]
% Dreieckskonstruktion
\coordinate[Punkt={left}{B}] (B) at (0,0);
\coordinate[Punkt={below}{C}] (C) at (4,0);
\draw[] (B) -- (C);
% Mittelsenkrechte
\coordinate[Punkt={below}{X}] (X) at ($(C)!0.5!(B)$);
\draw[densely dashed, shorten >=-5mm] (X) -- ($(X)!3cm!90:(C)$) coordinate[Punkt={right}{P}] (P);
% Winkel
\draw[] (B) --+ (90-\Alpha:\r) coordinate[Punkt={right}{Z}] (Z);
% Winkel
\draw pic [draw, angle radius=7mm, %angle eccentricity=1.3,
pic text={\phantom{.}}, pic text options={
pin={[pin edge={red,shorten <=-5pt}]95:{$90^\circ$-$\alpha$}}},
double] {angle =X--B--Z};
\end{tikzpicture}
\end{document}