$${\mathtt{A}} = {\frac{{\mathtt{a}}{\mathtt{\,\times\,}}{\mathtt{ha}}}{{\mathtt{2}}}}$$ $${\mathtt{A}} = {\frac{{\mathtt{b}}{\mathtt{\,\times\,}}{\mathtt{hb}}}{{\mathtt{2}}}}$$ $${\mathtt{A}} = {\frac{{\mathtt{c}}{\mathtt{\,\times\,}}{\mathtt{hc}}}{{\mathtt{2}}}}$$
http://rechneronline.de/pi/dreieck.php
http://mathe-abakus.fraedrich.de/mathematik/grzahlen.html

http://www.frustfrei-lernen.de/mathematik/koordinatensystem-2d-3d-abszisse-ordinate-mathematik.html