Multiple choice

O is the circumcentre of ΔABC, given ∠BAC = 85o and ∠ BCA = 55o , find ∠OAC.

  1. 40o

  2. 50o

  3. 45o

  4. 90o
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

First find ∠B = 180° - 85° - 55° = 40°. Since O is the circumcenter, ∠AOC = 2∠B = 80° (central angle theorem). In triangle AOC, OA = OC (radii), so base angles are equal: ∠OAC = ∠OCA = (180° - 80°)/2 = 50°.

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