Q:

How to draw the triangle in LaTeX?

I know how tow draw the: 
\triangle{ABC}
it will show

but how can I draw the:

A:

A PSTricks solution only for either fun or comparison purposes. Compile it with xelatex or latex-dvips-ps2pdf. 
\documentclass[pstricks,12pt]{standalone}
\usepackage{pst-eucl}
\begin{document}
\pspicture(8,6)
\pstTriangle(1,1){A}(7,1){C}(7,5){B}
\pstRightAngle{A}{C}{B}
\endpspicture
\end{document}

