The measure of an exterior angle
is equal to the sum
of the remote interior
Givens are angles a, b, c, 1, 2, 3.
\a b / 2
- 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).