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Class 7
Chapter 7: A Tale of Three Intersecting Lines
The three angles of any triangle add up to 180°. Triangles can be classified by sides (scalene, isosceles, equilateral) or by angles (acute, right, obtuse).
Learning outcomes
- Classify triangles by sides and angles
- Apply the angle-sum property of triangles
Activities
- Cut-and-paste angle-sum proof
Worked examples
Read through these first, then try the practice below.
Example 1 — Find the missing angle
A triangle has angles 50° and 70°. What is the third angle?
Solution: 180 − (50 + 70) = 60°.
Example 2 — Classify by sides
A triangle has sides 5, 5 and 7 cm. By sides, what is it called?
Solution: Two sides equal — isosceles.
Self-do practice
Question 1 of 3 · Score 0/0Two angles of a triangle are 40° and 60°. What is the third?
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