Section 5.3: Triangles
Essential Ideas

  • Congruent Triangles
    Two triangles are congruent when they have the same size and shape. There are six parts to a triangle: three angles and three sides.

  • A new construction is introduced in this section, namely constructing a triangle congruent to a given triangle.

  • Sum of the Measures of the Angles in a Triangle
    The sum of the measures of the angles in any triangle is 180 °.

  • Exterior Angle Formula
    The measure of the exterior angle of a triangle equals the sum of the measures of the two opposite interior angles.