\begin{tikzpicture}[font=\small]
  \def\couleur{orange}
  \path[coordinate] (0,0) coordinate(A)
              ++( 90:4cm) coordinate(B)
              ++(0:4cm) coordinate(C)
              ++(-90:4cm) coordinate(D);
  \draw (A) node[left] {A}
    (B) node[left] {B}
    (C) node[right] {Point C} --
    (D) node[right,midway,align=left] {边长$a$ \\ 面积和：$S=\frac{50}{9}a^2$ \\ 边长和：$C=\frac{20}{9}(10+\sqrt{82})a$}
    node[right] {点D};
  \draw[fill=\couleur!\thedensity] (A)--(B)--(C)--(D)--cycle;
  \foreach \x in {1,...,40}{%
      \pgfmathsetcounter{density}{\thedensity+25}
      \setcounter{density}{\thedensity}
      \path[coordinate] coordinate(X) at (A){};
      \path[coordinate] (A) -- (B) coordinate[pos=.1](A)
                          -- (C) coordinate[pos=.1](B)
                          -- (D) coordinate[pos=.1](C)
                          -- (X) coordinate[pos=.1](D);
      \draw[fill=\couleur!\thedensity] (A)--(B)--(C)--(D)--cycle;
  }
\end{tikzpicture}
