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Q.

If α,β and γ are three distinct real values such that sinα+sinβ+sinγsin(α+β+γ)=cosα+cosβ+cosγcos(α+β+γ)=2 then  cos(α+β)+cos(β+γ)+cos(γ+α)=___

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a

2

b

3

c

4

d

5

answer is A.

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Detailed Solution

put , a=cis α,b=cisβ, c=cisγ
a+b+c=2abc 1ab=2
 cosα+β=2
sinα+sinβ+sinγsin(α+β+γ)=cosα+cosβ+cosγcos(α+β+γ)
sinα+β+γsinα+sinβ+sinγ+cosα+β+γcosα+cosβ+cosγsin(α+β+γ)sinα+β+γ+cosα+β+γcosα+β+γ
ab=cd=xa+ycxb+yd (By ratio and proportion)
cosα+β+cosβ+γ+cosγ+α1=2

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