For a regular triangle, .

The measure of an interior angle of a regular n-gon formula is

Interior angle       (Substitute 3 for n)

                                             (Subtract numarator: )

                                                 (Divide: )

In a regular triangle each interior angle forms a straight angle with exterior angle at each

vertex.

So, interior and exterior angles are suplementary angles.

Interior angle + exterior angle =      (Angles are supplementary)

                     (Substitute interior angle )  (Subtract from each side)

Aplpy commutative property of addition: .

           (Apply additive inverse property:)

                   (Apply additive identity property) \ \

                              (Subtract: )

The measure of an interior angle of a regular triangle is .

The measure of an exterior angle of a regular triangle is .