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0580/12 Feb/Mar 2025, Question 15: Angles in a Triangle

Cambridge IGCSE Mathematics 0580 · Paper 12 (Core) · Non-calculator · 5 marks

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The question, in short

Triangle BCD has DB = DC = BC and lies on straight line ABC with angle y at B; name the triangle and find y with two geometrical reasons.

Summarised in our own words. Use the official paper for the exact wording and the diagram.

Part (a) · 1 mark

State the mathematical name for triangle BCD, given all three sides are equal.

  1. 1All three sides are equal, so the triangle is equilateral

Answerequilateral

Part (b) · 4 marks

Calculate the value of y and give two geometrical reasons supporting your answer.

  1. 1Angles in an equilateral triangle are equal, so angle DBC = 180 ÷ 3 = 60°
  2. 2Angles on a straight line add to 180°
  3. 3y = 180 − 60 = 120

Answery = 120; angles in an equilateral triangle are equal; angles on a straight line add to 180

Stuck on a step? Read the free lesson: Angles in a Triangle

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