While each side has one point, an angle named DEG has the points D and G as its sides and E as the vertex point. Angle 2 and 3 make a straight angle. Angle 3 and 4 make a straight angle. AB and CD are intersecting lines and intersection point is O. So 4 angles are formed. Angles ∠1 and ∠3 form a pair of vertically opposite angles; while ∠2 and ∠4 form another pair of vertically opposite angles. Theorem 1-6-1: Pythagorean Theorem: In a right triangle, the vertex is the middle point that unites both sides of the rays. For example, the sum of the squares of the lengths of the legs is equal to the square of the length of the hypotenuse.