Perhaps you were looking at $\displaystyle\angle{G}{D}{F}\usepackage{cmbright}\Huge\$ , which is a straight angle, and saw that these two angles were parts of it. However, they are not all of it. Supplementary angles are two angles whose measurements add up to $\displaystyle{{180}}^{\circ}\usepackage{cmbright}\Huge\$ , or that together make an entire straight angle.