 Quick question. How many exterior angles does a triangle have? I've listed out 12 of these angles, but not all of them are exterior angles. We define an exterior angle as one that's made by extending one side of a triangle. In other words, angle 2 is an exterior angle because we just extended this one side. Angle 9 is also an exterior angle because it exists by extending this other side. However, angle 1 is not an exterior angle. Likewise, angle 7 is not and 4 is not. So how many exterior angles does a triangle have? Well, 2 and 9, 8, 6, 3 and 5 are all exterior angles, so 6 of them.