The measure of an

exterior angle is equal to the

sum of the

remote interior angles.

Proof:

Givens are angles a, b, c, 1, 2, 3.

`
\
\
\1______________________
\a b / 2
\ /
\ /
\ /
\ /
\ /
\ /
\c /
\/
3 /
/
/
`

- a+b+c = 180° (by sum of angles of triangle)
- b+c = 180-a (by subtraction)
- a+1 = 180° (by linear pair postulate)
- 1 = 180-a (subtraction)
- 1 = b+c (by substitution).

Q.E.D.