[Tex/LaTex] Parallelogram with TikZ

tikz-pgf

How to improve the following?

\documentclass[tikz,border=7mm]{standalone}
\begin{document}
  \begin{tikzpicture}[xslant=1,xscale=3,scale=2]
    \draw (0,0) rectangle
          (1,1) coordinate(C) node[above]{C} -- (0,0) coordinate(A) node[below]{A}
          (1,0) coordinate(D) node[below]{y} -- (0,1) coordinate(B) node[above]{x};
  \end{tikzpicture}
\end{document}

I want to place x+y and x-y below the lines like in this picture:

enter image description here

Best Answer

With tkz-euclide v2.42b (beta) actually only here

%!TEX TS-program = lualatex
\documentclass{standalone}
\usepackage{tkz-euclide}

\begin{document}

\begin{tikzpicture}[scale=1.5]
    %initialisation
    \tkzInit[xmin=0,xmax=4,ymin=0,ymax=2] 
    \tkzClip[space=.5] 
    %definitions
    \tkzDefPoint(0,0){A} 
    \tkzDefPoint(3,0){B} 
    \tkzDefPoint(4,2){C} 
    \tkzDefPointWith[colinear= at C](B,A) \tkzGetPoint{D}
    %drawing
    \tkzDrawPolygon(A,B,C,D)
    \tkzDrawSegments[blue,dashed](A,C B,D)
    %label
    \tkzLabelPoints(A,B)
    \tkzLabelPoints[above right](C,D)
    \tkzLabelSegment[above,pos=.7,sloped](A,C){$x+y$}
    \tkzLabelSegment[above,pos=.7,sloped](B,D){$x-y$}
\end{tikzpicture}

 \end{document}

enter image description here