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Understanding and Classifying Triangles: Identifying Angles and Sides

In this lesson, you will learn how to identify and classify triangles based on their angles and side lengths. A triangle is defined as a figure with three sides and three angles, with the sum of the angles equaling 180 degrees. Triangles can be classified by their angles (acute, obtuse, right) and by their sides (scalene, isosceles, equilateral). You will also practice finding missing angle measures and classifying triangles through guided examples and exercises.

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Understanding and Classifying Triangles: Identifying Angles and Sides

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  1. Lesson 10-4Pages 428-431 Triangles

  2. What you will learn! How to identify and classify triangles.

  3. Vocabulary

  4. What you really need to know! A triangleis a figure with three sides and three angles. The sum of the measures of the angles of a triangle is 180. You can use this to find a missing angle measure in a triangle.

  5. Triangles can be classified by • the measures of their angles: • An acute triangle has three acute angles. • An obtuse triangle has one obtuse angle. • A right triangle has one right angle.

  6. Triangles can also be classified • by the lengths of their sides: • In a scalene triangle, all sides have different lengths. • An isosceles triangle has at least two congruent sides. • An equilateral triangle has all three sides congruent.

  7. What you really need to know! Sides that are the same length are congruent segmentsand are often marked by tick marks.

  8. Example 1: An airplane has wings that are shaped like triangles. Find the missing measure.

  9. 112 + 47 = 159 180 – 159 = 21 21°

  10. Example 2: Find m A in ∆ABC if m A = m B and m C = 80°. 180 = m A + m B + m C

  11. 180 = m A + m B + m C 180 = m A + m B + 80 100 = m A + m B 100 = 50 + 50 m A = 50°m B = 50°

  12. Example 3: Classify the triangle by its angles and by its sides. 120° 30° 30° Obtuse Isosceles

  13. Example 4: Classify the triangle by its angles and by its sides. 70° 80° 30° Acute Scalene

  14. Page 430 Guided Practice #’s 3-7

  15. Read: Pages 428-429 with someone at home and study examples!

  16. Homework: Page 430-431 #’s 8-24 all #’s 27-34 all Lesson Check 10-4

  17. Page 588 Lesson 10-4

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