\documentclass[tikz, border=10pt,convert=true]{standalone}
\usepackage[]{tkz-euclide}
%\usetikzlibrary{shapes.geometric,calc}

\begin{document}
\begin{tikzpicture}[scale=3]
    \tkzDefPoints{0/0/M}
    \tkzDefShiftPoint[M](40:2){A}    % polar coord rel.to M: 40deg, r=2 
    \tkzDefShiftPoint[M](-40:2){C}   % rel. to M: -40deg, r=2
    \tkzDefShiftPoint[M](0:2){B}     %  just 2 steps to the right
    \tkzInterLL(A,C)(M,B)
        \tkzGetPoint{N}              % intersection of lines
    \tkzDrawArc[delta=10,very thick](M,C)(A)  % arc 
    \tkzDrawPoints(M,A,B,C,N)
    \tkzLabelPoints[below](M,C)
    \tkzLabelPoints[above right](A)
    \tkzLabelPoints[right](B)
    \tkzLabelPoints[below left](N)

    \tkzDrawSegment[green, very thick](A,B)
    \tkzDrawSegment[green, very thick](B,C)
    \tkzDrawSegment[blue, very thick](A,C)
    \tkzDrawSegment[black, very thick](M,B)
    \tkzDrawSegment[black, very thick](M,A)
    \tkzDrawSegment[black, very thick](M,C)

    \tkzLabelSegment[above,pos=.5](M,A){$1$}
    \tkzLabelSegment[above,pos=.5](M,C){$1$}
    \tkzMarkRightAngle[size=.08](M,N,A)
    \tkzLabelSegment[above,pos=.5](M,N){$a$}
    \tkzLabelSegment[above,pos=.5](N,B){$1-a$}
    \tkzLabelSegment[below right,pos=.45](A,B){$s_{2n}$}
    \tkzLabelSegment[left,pos=.4](A,C){$s_{n}$}
    \tkzLabelSegment[below left,pos=.3](B,C){$s_{2n}$}

\end{tikzpicture}

\end{document}

