Chapter 4.1 Notes: Apply Triangle Sum Properties Goal: You will classify triangles and find measures of their angles. • A triangle is a polygon with three sides. • A triangle with vertices A, B, and C is called “triangle ABC” or “∆ABC.” Classifying Triangles by Sides • A scalene triangle is a triangle with no congruent sides. • An isosceles triangle is a triangle with at least two congruent sides. • An equilateral triangle is a triangle with three congruent sides. Classifying Triangles by Angles • An acute triangle is a triangle with three acute angles. • A right triangle is a triangle with one right angle. • An obtuse triangle is a triangle with one obtuse angle. • An equiangular triangle is a triangle with three congruent angles. Ex.1: Classify the triangular shape of the support beams in the diagram by its sides and by measuring its angles. Ex.2: Classify the triangle shown in the diagram by its sides and angles. Ex.3: Classify the triangle by its sides and angles. Angles • When the sides of a polygon are extended, other angles are formed. • The original angles are the interior angles. • The angles that form linear pairs with the interior angles are the exterior angles. • Theorem 4.1 Triangle Sum Theorem: The sum of the measures of the interior angles of a triangle is 180o. • Theorem 4.2 Exterior Angle Theorem: The measure of an exterior angle of a triangle is equal to the sum of the measures of the two nonadjacent interior angles. Ex.4: Find . Ex.5: Find the measure of in the diagram shown. • A corollary to a theorem is a statement that can be proved easily using the theorem. • Corollary to the Triangle Sum Theorem: The acute angles of a right triangle are complementary. Ex.6: The tiled staircase shown forms a right triangle. The measure of one acute angle in the triangle is twice the measure of the other. Find the measure of each acute angle. Ex.7: Find Ex.8: Find the measure of each interior angle of ∆ABC, where Ex.9: Find the measures of the acute angles of the right triangle in the diagram shown.
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