GCSE Maths (AQA)

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Interior Angles in a Triangle

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Geometry and Measures Properties and Constructions

Interior Angles in a Triangle

3:46 Angles in Shapes
Spec G3 G6
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  • Interior angles in a triangle add up to 180 degrees
  • To prove this, add a line parallel to one side of the triangle at the opposite vertex and label the missing angles on the line
  • Use alternate angles to relate the angles on the line to the interior angles of the triangle
  • Write down the angle fact for angles on a straight line, which sum to 180 degrees
  • Use this fact to substitute the missing angles on the line with the interior angles of the triangle
  • The sum of the interior angles of a triangle is x + y + z = 180 degrees
  • Example: Calculate the value of angle x in a triangle with interior angles of 35 degrees, 45 degrees, and x. The equation is 35 + 45 + x = 180, so x is 100 degrees.

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