\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{backgrounds}
\usepackage{tkz-euclide}
\usetikzlibrary{angles,quotes}

\usetkzobj{all}
\usetikzlibrary{shapes, calc, decorations}
\usepackage{amsmath,amssymb}
\usepackage{helvet}
\usepackage[eulergreek]{sansmath}

\begin{document}
\begin{preview}
\begin{tikzpicture}[very thick,font=\sansmath\sffamily,x=3cm,y=3cm,  show background rectangle,
  background rectangle/.style={fill=white},]
    \tkzInit[xmax=1.0,ymax=1.0,xmin=-1.0,ymin=-1.0]
    \tkzGrid
    \tkzAxeXY[up space=0.2,right space=0.2,fill=none]

    \tkzDefPoint(0,0){A}
    \tkzDefPoint({cos(FPpi/5)*1},0){B}
    \tkzDefPoint({cos(FPpi/5)*1},{sin(FPpi/5)*1}){C}
    \tkzDefPoint(1,0){CircleEnd}

    \tkzDrawCircle(A,CircleEnd)

    % \tkzLabelPoints[below left,font=\sansmath\sffamily](A)
    % \tkzLabelPoints[below right,font=\sansmath\sffamily](B)

    % Mark angle
    \tkzMarkAngle[arc=l,size=0.6cm,fill=green!30](B,A,C)
    \tkzLabelAngle[pos=0.15](B,A,C){$\alpha$}

    % Draw polygon
    \tkzDrawPolygon(A,B,C)
    \tkzDrawSegment[black!60!green,very thick](A,B)
    \tkzDrawSegment[red,very thick](B,C)

    % Label lines
    \tkzLabelSegment[above](A,C){$1$}
    \tkzLabelSegment[sloped,above=-0.05,red](B,C){$\sin(\alpha)$}
    \tkzLabelSegment[below,black!60!green](A,B){$\cos(\alpha)$}
\end{tikzpicture}
\end{preview}
\end{document}