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in Trigonometry by (45 points)
If (a+b+c)(b+c-a)=3bc,show that A=60°.

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1 Answer

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by (15 points)

(b+c+a)(b+c-a)

=(b+c)2 - a= 3bc

b+ c- a= bc

(b2 + c- a2)/2bc  =  1/2

So,  Cos A = 1/2.        (Cosine rule)

So, A = 60

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