(3) The third angles of two triangles are equal if the two angles of one triangle are equal to

the two angles of the other triangle.

(4) The right triangles are equal if the side and the acute angle of one triangle are equal to the

corresponding side and the acute angle of the other triangle.

(5) Any exterior angle of a triangle is equal to the sum of the two opposite interior angles.

b. Theorem 2. Any side of a triangle is less than the sum of the other sides. This theorem can

be proved by using the postulate in paragraph 3-6a. As shown in Figure 3-20, side AB is a straight line

and the other two sides, AC and CB, form a bent line between points A and B.

c. Theorem 3. If two sides of a triangle are equal, the angles opposite these sides are equal, as

shown in Figure 3-21. Conversely, if two angles of a triangle are equal, the sides opposite these angles

are equal. The following corollaries can be derived from this theorem: