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Learn how to classify triangles by sides and angles, calculate missing angles, explore interior and exterior angles, and understand important theorems related to triangles.
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Classifying Triangles Angle Measures of Triangles
Triangle • A triangle is a figure formed by three segments joining three noncollinear points.
Classifying Triangles by Sides • Equilateral Triangle • Isosceles Triangle • Scalene Triangle
Equilateral Triangle • An equilateral triangle has three congruent sides.
Isosceles Triangle • An isosceles triangle has at least two congruent sides.
Scalene Triangle • A scalene triangle has no congruent sides.
Classification of Triangles by Angles • Equiangular triangle • Acute triangle • Right triangle • Obtuse triangle
Equiangular Triangle • An equiangular triangle has three congruent angles.
Acute Triangle • An acute triangle has three acute angles.
Right Triangle • A right triangle has one right angle.
Obtuse Triangle • An obtuse triangle has one obtuse angle
Vertex • A vertex of a triangle is a point that joins two sides of the triangle. • The side across from an angle is the opposite side.
Name the side that is opposite the angle. • Angle J • Angle K • Angle L
Triangle Sum Theorem • The sum of the measures of the angles of a triangle is 180º. • In ΔABC, mA + m B + m C = 180º
Corollary to the Triangle Sum Theorem • The acute angles of a right triangle are complementary. • In ΔABC, if m C = 90º, then m A + m B= 90º A B C
Interior Angles • When the sides of a triangle are extended, other angles are formed. • The three original angles are the interior angles.
Exterior Angles • The angles that are adjacent to the interior angles are the exterior angles. • It is common to show only one exterior angle at a vertex.
Exterior Angles Theorem • The measure of an exterior angle of a triangle is equal to the sum of the two remote interior angles. • m 1 = m A + m B