Geometry and measuresKey Stage 3

Angles in an isosceles triangle

Use equal base angles and the 180° angle sum.

  • The angles opposite the equal sides are equal.
  • If the vertex angle is known, subtract it from and divide by .
  • If a base angle is known, double it before subtracting from .

Example 1

Find angle in the isosceles triangle.

Subtract the vertex angle, then share equally between the base angles.

Example 2

Find angle in the isosceles triangle.

The base angles are equal.Angles in a triangle add to 180°.

Your turn

Question 1

Find angle in the isosceles triangle.

Check answer
Subtract the vertex angle, then share equally between the base angles.
Question 2

Find angle in the isosceles triangle.

Check answer
The base angles are equal.Angles in a triangle add to 180°.
Question 3

Find angle in the isosceles triangle.

Check answer
Subtract the vertex angle, then share equally between the base angles.
Question 4

Find angle in the isosceles triangle.

Check answer
The base angles are equal.Angles in a triangle add to 180°.
Question 5

Find angle in the isosceles triangle.

Check answer
Subtract the vertex angle, then share equally between the base angles.
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