Isosceles & Equilateral Triangles | Geometry Angle & Side Relationships

In this Geometry lesson, we explore the special properties of *isosceles and equilateral triangles* and learn how to use relationships between congruent sides and angles to solve problems. You'll learn how to identify base angles and vertex angles, use the Isosceles Triangle Theorem and its converse, and solve algebraic equations involving triangle side lengths and angle measures. *After watching this lesson, you should be able to:* Identify congruent sides and angles using matching marks. Recognize the base angles and vertex angle of an isosceles triangle. Explain why congruent sides have congruent opposite angles. Apply the converse: congruent angles have congruent opposite sides. Find missing angles using the Triangle Angle Sum Theorem. Solve equations involving angle expressions and side lengths. Explain why every angle in an equilateral triangle measures 60°. Find missing side lengths and perimeters of special triangles. *Key Vocabulary:* Isosceles Triangle • Equilateral Triangle • Congruent Sides • Congruent Angles • Base Angles • Vertex Angle • Converse • Perimeter *Important Relationships:* Isosceles Triangle: Two congruent sides → two congruent opposite angles. Two congruent angles → two congruent opposite sides. Equilateral Triangle: Three congruent sides. Three congruent angles. Each interior angle measures 60°. Triangle Angle Sum: m∠A + m∠B + m∠C = 180° 📓 *While Watching:* WATCH → WRITE → TRY → QUESTION 1. Watch the lesson and complete your Geometry notebook notes. 2. Pay attention to matching side and angle markings. 3. Pause the video and attempt each example before viewing the solution. 4. Complete the “STOP THE VIDEO — YOUR TURN” problem independently. 5. Bring any questions to class. *Remember:* The location of the vertex angle depends on the congruent sides, not on how the triangle is oriented. Come prepared to apply these relationships to more challenging triangle problems!