Geometry and measuresKS3 / GCSE

Angles in a triangle

Find a missing interior angle, including when a side is extended.

  • The three interior angles add to .
  • An exterior angle and its adjacent interior angle add to .
  • Find an unknown interior angle first, then use the triangle total.

Example 1

Find angle .

Angles in a triangle add to 180°.

Example 2

Find angle .

First use the straight line to find the interior angle.Now use the three interior angles.

Your turn

Question 1

Find angle .

Check answer
Angles in a triangle add to 180°.
Question 2

Find angle .

Check answer
First use the straight line to find the interior angle.Now use the three interior angles.
Question 3

Find angle .

Check answer
Angles in a triangle add to 180°.
Question 4

Find angle .

Check answer
First use the straight line to find the interior angle.Now use the three interior angles.
Question 5

Find angle .

Check answer
Angles in a triangle add to 180°.
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