In the given figure, O is the centre of the circle. If BD = DC and ∠DBC=30∘, find the measures of ∠BAC and ∠BOC.
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Solution
BD = DC So, ΔBDC is an isosceles triangle BD = DC ∠DBC=∠DCB=30∘ ∴∠BDC=120∘ (angle seen property of Δ) ABDC is a cyclic quadrilateral So, ∠BAC=180∘−120∘=60∘ (opposite angle is supplementary in cyclic quadrilateral) ∠BOC=2×∠BAC=2×60∘ =120∘ (Angle subtend at centre is twice of angle subtend remaining part of circle)