Answer:
5 cm
Step-by-step explanation:
ΔABC is equilateral, so BC = AB = 5 cm.
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AO is the perpendicular bisector of BC and the angle bisector of angle A, so ∠OAC = ∠OAB = 30°. Then vertex angle BAC of isosceles triangle ABC is 60°, which means all angles in the triangle are 60° and the triangle is equilateral.
