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. (d
^{o}> b^{o}, d^{o}> c^{o})

## 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

**(d**^{o}** > b**^{o}**, d**^{o}** > c**^{o}**)** :

Because d = b + c, where **b **and **c **are positive (in triangle) so we can say

d^{o} > b^{o}

d^{o} > c^{o}