An architect designs a circular window with centre O. Points A, B and C lie on the circumference. The angle CAB = 35° and both angles are subtended by arc BC. What is the size of angle COB?

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Question

An architect designs a circular window with centre O. Points A, B and C lie on the circumference. The angle CAB = 35° and both angles are subtended by arc BC. What is the size of angle COB?

Accepted Answer

70° — Award 1 mark for applying the theorem that the angle at the centre is twice the angle at the circumference when both are subtended by the same arc. Angle COB = 2 × 35° = 70°. A is incorrect — this halves instead of doubles. B is incorrect — this confuses the angle at the circumference with the angle at the centre. D is incorrect — this is 180° − 35°, incorrectly applying supplementary angles.

Answer

35°

Answer

17.5°

Answer

145°

Answer

70°

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