16 For each vertex we have a linear pair, so: measure of EXTERIOR ANGLE + measure of INTERIOR ANGLE =180° substracting measure of INTERIOR ANGLE from both sides: measure of EXTERIOR ANGLE=180°-measure of INTERIOR ANGLE Multiplying both sides for n=number of vertices: n(measure of EXTERIOR ANGLE)=n(180°-measure of INTERIOR ANGLE) n(measure of EXTERIOR ANGLE)=n180°-n(measure of INTERIOR ANGLE) n180° -EXTERIOR ANGLE SUM=INTERIOR ANGLE SUM = EXTERIOR ANGLE SUM n180° - 180°(n-2) EXTERIOR ANGLE SUM= n180°-n180°+360° EXTERIOR ANGLE SUM= 360° Standard 12 Click for a formal definition…