# The Problem!

Let ABC be a triangle in which AB = AC and let I be its in-centre. Suppose BC = AB + AI. Find ∠BAC.

- For any triangle ABC, \( \frac{\sin A}{a} = \frac{\sin B } {b} = \frac {\sin C }{c} \).
**Addendo:**If \( \frac{a}{b} = \frac{c}{d} \) then each of these ratios are equal to \( \frac {a+b}{c+d} \)- \( \sin (90 + \theta ) = \cos \theta \)
- \( \cos ^2 2 \theta = 2 \cos ^2 \theta -1 = \cos ^2 \theta – \sin ^2 \theta \)

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