documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}
coordinate (A) at (0, 0);
coordinate (B) at (4, 0);
coordinate (C) at (4, 3);
draw (A) -- (B) -- (C) -- cycle;
end{tikzpicture}


begin{tikzpicture}
coordinate (A) at (0, 0);
coordinate (B) at (4, 0);
coordinate (C) at (4, 3);
draw (A) -- (C);
draw (A) -- (B);
draw (B) -- (C);
end{tikzpicture}
end{document}