Sine law and circumscribed circle

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How is $\frac{a}{\sin(A)}=2R$ (where $R$ is the radius) derived?

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I trust you know that $\angle BDC=2 \angle A $

Construct an isosceles $\triangle BDC $.

Drop perpendicular from D to BC.

See that $a=2R \cdot \sin (\frac {1}{2} \angle BDC) $

Substitute the first equality I mentioned