The given interior angles at vertex A , B and C are respectively.

At each vertex each interior angle forms a straight angle with exterior angle.

So, interior and exterior angles are supplementary angles.

At vertex A:

          (Angles are supplementary)                             (Interior angle at A is )          (Subtract from each side)

Apply commutative propetry of addition: .

\ \

       (Apply additive inverse property: ) \ \

                (Apply additive inverse property: )

                            (Subtract: )

At vertex B:

          (Angles are supplementary)                           (Interior angle at A is )    (Subtract from each side)

Apply commutative propetry of addition: .

       (Apply additive inverse property: )

             (Apply additive identity property) \ \

                              (Subtract: )

At vertex C:

          (Angles are supplementary)                              (Interior angle at A is )          (Subtract from each side)

Apply commutative property of addition:.

           (Apply additive inverse property: )

                   (Apply additive identity property)

                              (Subtract: )

The exterior angles at vertex A , B and C are respectively.