Fundamental Ideas
Triangles
With the use of the Parallel Postulate, the following theorem can be proven.
Theorem 25: The sum of the interior angles of any triangle is 180°.
m ∠ A + m ∠ B + m ∠ C = 180°.
Example 1: If m ∠ A = 40° and m ∠ B = 60°, find m ∠ C.
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- The Triangle Defined
- Angle Sum of a Triangle
- Exterior Angle of a Triangle
- Classifying Triangles by Sides or Angles
- Special Names for Sides and Angles
- Altitudes, Medians, and Angle Bisectors
- Congruent Triangles
- Special Features of Isosceles Triangles
- Triangle Inequalities: Sides and Angles
- The Triangle Inequality Theorem
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Angle Sum of a Triangle
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