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Give a Reason / Justify

Afrikaans: Gee 'n rede / Motiveer

State the specific theorem, axiom, or rule that makes your answer correct. In Euclidean Geometry this means naming the exact theorem (for example: 'angles subtended by the same chord in the same segment are equal'). In algebra or arithmetic it means citing the property you applied. A calculation that produces the right number does not justify the claim, the justification is the rule, not the arithmetic.

Common mistake

Writing a calculation or restating the answer instead of naming the mathematical reason. Saying 'angle ABC = 50° because angle DEF = 50°' is a restatement, not a reason. The reason must be the geometric theorem that allows you to conclude the angles are equal. In Euclidean Geometry, every angle or side you introduce should be accompanied by its reason in brackets, missing reasons on a step that earns a mark will cost you that mark.

Signal phrases

These are the words that tell you this instruction is being used.

  • Give a reason for your answer
  • Justify your statement
  • Give reasons for ALL statements
  • State the theorem used

Past paper examples

Give a reason why ∠ABC = ∠ADC.

Source: DBE Mathematics Paper 2, November 2023, Question 9.1.2

Test yourself

Before you read the answer, what is this question asking you to do?

O is the centre of the circle. Angle AOB = 80°. Angle ACB is an angle at the circumference subtended by the same arc AB. Calculate the size of angle ACB and give a reason.

Practice question: not from a past paper

Where this comes up