The measure of an exterior angle of a triangle
is equal to the sum of the measures of its two remote interior angles.
/\
/ 1\
/ \
/ \
_____2/3____4\
1. / 1 + / 3 + / 4 = 180 1. Triangle Sum Theorem.
2. / 2 is supplementary to / 3 2. Linear Pairs are supplementary.
3. / 2 + / 3 = 180. 3. Definition of supplementary.
4. / 1 + / 3 + / 4 = / 2 + / 3 4. Substitution.
5. / 1 + / 4 = / 2 5. Subtraction.