Geometry Postulates and Theorems textbook tutorials - definitions - constructions - postulates and theorems - internet activities and resources mathematical logic symmetry and similarity points, lines and planes parallel lines triangles quadrilaterals polygons circles 3-Dimensional Geometry Related Thorems Polygon Theorems The angles that form a linear pair are supplementary. In a polygon with n sides the sum of the angle measures is (n - 2) 180. Vertical angles are congruent. Exterior Angles of a Regular Polygon The sum of the measures of the exterior angles of any polygon is 360. Two lines are perpendicular if and only if they form congruent adjacent angles.   The Triangle Sum Theorem The sum of the angle measures of a trianlge is 180.           The opposite sides of a parallelogram are congruent.   The opposite angles of a parallelogram are congruent.   The diagonals of a parallelogram bisect each other.