 T RIANGLE : A figure formed by three noncollinear points, connected by segments  Since two of the segments create a vertex, a triangle has three vertices. E D F angle side vertex

 The sides are DE, EF, and DF  The vertices are D, E, and F  The angles are D, E, and F E D F angle side vertex

 In chapter 3, we identified angles as acute, obtuse or right. Triangles are also classified by their angles.  All triangles have at least two acute angles. The third angle is either acute, right, or obtuse.

 Triangles can also be classified by their sides.  Since all sides of an equilateral triangle are congruent, then at least two of its sides are congruent. This means all equilateral triangles are also isosceles triangles.

 Isosceles triangles have special names for some of their parts.

 Your Turn: Classify the triangle by its angles and sides.  Right isoscelesAcute equilateral

 Your Turn: Classify the triangle by its angles and sides.  Obtuse scaleneRight scalene

 Find the measures of AB and BC of isosceles triangle ABC if A is the vertex angle. › If A is the vertex angle, then AB  AC. Therefore, AB = 23. › To find BC, we first need to determine x. We can use the two equal sides to accomplish this.  5x – 7 = 23  5x = 30 (add 7 each side)  x = 6 (divide each side by 5)  BC = 3(6) – 5  BC = 18 – 5  BC = 13

 Assignment: Worksheet #5-1