triangles

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
  % Give three nodes names
  \coordinate (a) at (90:3cm);
  \coordinate (b) at (210:3cm);
  \coordinate (c) at (-30:3cm);

  % Label those nodes
  \node [above] at (a) {$a$};
  \node [below left] at (b) {$b$};
  \node [below right] at (c) {$c$};

  % The below transform canvas shenanigans require the following explicit
  % instruction about the bounding box of the figure
  \path[use as bounding box] (210:4cm) rectangle (7cm,7cm);

  % Within the scope, the x,y,z components are treated like barycentric coordinates
  \begin{scope}[x=(a),y=(b),z=(c)]
    % Draw outer triangle
    \draw (a) -- (b) -- (c) --cycle;
    % Draw dashed line
    \draw [dashed] (1,0,0) -- (0.4,0.3,0.3);
    % Draw red filled shape
    \fill[fill=red, opacity=0.5] (0.4,0.3,0.3) -- (0,0.3,0.7) -- (0,0.7,0.3)--cycle;

    % Use a copy of the same coordinate system but shifted up and to the right
    \begin{scope}[transform canvas={xshift=4cm,yshift=3.5cm}]
      \draw[blue] (a) -- (b) -- (c) --cycle;
    \end{scope}
  \end{scope}
  
  % Draw the curved arrow
  % `tips=proper' is required for some reason...
  % I had to sort of just guess the arrow start and end points...
  \draw [-stealth,line width = 5pt,tips=proper](75:3cm) edge [bend left] ($(75:3cm) + (1cm,1cm)$);
\end{tikzpicture}
\end{document}