1. Lesson 2.1 Classifying Triangles Triangle- a closed figure in a plane that is the union of three segments endpoint to endpoint. Interior Angle- an angle withing the triangle. A triangle has three interior angles. Exterior Angle- an angle formed outside the triangle by one side and the extension of the adjacent side. Classifying Triangles According to Sides Scalene Triangle- a triangle that has no congruent sides. Isosceles Triangle- a triangle that has two congruent sides. Legs- the congruent sides of an isosceles triangle. Base- the third side of an isosceles triangle. Base angles- the angles whose vertices are the endpoints of the base. Vertex angle- the angle opposite the base Equilateral Triangle- a triangle that has three congruent sides.

2. Classifying Triangles According to Angles Acute Triangle- a triangle that has three acute angles. Equiangular Triangle- a triangle that has three congruent angles. Right Triangle- a triangle that has one right angle. Legs- the sides of the triangle that form the right angle. Hypotenuse- the side opposite the right angle. Obtuse Triangle- a triangle that has an obtuse angle. Example 1: In DEF, m<E = 90 and EF = ED. a. Classify the triangle according to sides. b. Classify the triangle according to angles. c. What sides are the legs of the triangle? d. What side is opposite the right angle? e. What angle is opposite EF? f. What angle is included between EF and FD

3. Theorem- The sum of the measures of the angles of a triangle is 180. Example 2: The measure of the vertex angle of an isosceles triangle exceeds the measure of each baseangle by 30 degrees. Find the measure of each angle of the triangle. Example 3: In ABC, the measures of the three angles are represented by 9x, 3x - 6, and 11x + 2. Showthat ABC is a right triangle. HW Pg 27-28/ 1-18 all