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Question
In ∆ABC, if b2 + c2 − a2 = bc, then ∠A = ______.
Options
`pi/4`
`pi/3`
`pi/2`
`pi/6`
Solution
In ∆ABC, if b2 + c2 − a2 = bc, then ∠A = `bbunderline(pi/3)`.
Explanation:
By cosine rule,
`cos A = (b^2 + c^2 - a^2)/(2 bc)`
∴ `cos A = (bc)/(2 bc)` ...[b2 + c2 − a2 = bc]
∴ `cos A = 1/2`
∴ `angle A = pi/3`
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