Primary 6 · Angles · Question type 8.2

P6 Angles: Angles in an Isosceles Triangle

Two equal sides give two equal angles. Subtract the third angle from 180°, then divide the remainder by 2.

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Primary 6, on the MOE Primary Mathematics syllabus.

In the Angles chapter8 question types · see the chapter

The lesson · 1 minute 50 seconds

How do you find angles in a P6 isosceles triangle?

Watch: an isosceles triangle with 76° at the top

Another worked example

A worked example

ABC is a right-angled triangle with the right angle at B. D lies on AC and BD = AD, marked with ticks. Angle ACB = 24°. Find angle BDC.

The working · Four lines

The step-by-step working

Written out in full, the way it should appear on paper.

∠BAC = 180° - 90° - 24° = 66° (angle sum of triangle ABC)
∠DBA = ∠DAB = 66° (base angle of isosceles triangle ABD)
∠ADB = 180° - 66° - 66° = 48° (angle sum of triangle ABD)
∠BDC = 180° - 48° = 132° (angles on a straight line ADC)

The answer is 132°.

Practise · Modelled on top school exam questions

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Try it yourself

Complete a right-angled triangle

Use the right angle and the triangle angle sum to find the unknown angle.

TUV is a right-angled triangle with the right angle at T. Angle TVU = 38°. Find angle TUV.

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∠TUV = 180° - 90° - 38° = 52° (angle sum of triangle TUV)

Ans: 52°

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Next, use the marks on a quadrilateral to choose a property. Angles in a Composite Figure →

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