Exterior Angle Theorem For a triangle

- The exterior angle d equals the angles b plus c. (d = b + c)
- The exterior angle d is greater than angle b and angle c. (do > bo, do > co)
Proof
d = b + c :
a + b + c = 180° (Because the interior angles of a triangle add to 180°)
a + d = 180° (Angles a and d make a straight angle, which is 180°)
So
a + d = a + b + c
d = b + c
(do > bo, do > co) :
Because d = b + c, where b and c are positive (in triangle) so we can say
do > bo
do > co