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Questions  

 If cosA+cosB+cosC=0 and cos3A+cos3B+cos3C=λcosAcosBcosC then λ is 

a
8
b
10
c
12
d
16

detailed solution

Correct option is C

If cos⁡A+cos⁡B+cos⁡C=0 then cos3⁡A+cos3⁡B+cos3⁡C=3cos⁡Acos⁡Bcos⁡C Given cos⁡3A+cos⁡3B+cos⁡3c=λcos⁡Acos⁡Bcos⁡C4cos3⁡A−3cos⁡A+4cos3⁡B−3cos⁡B+4cos3⁡C−3cos⁡C=λcos⁡Acos⁡Bcos⁡C=4cos3⁡A+cos3⁡B+cos3⁡C−3(cos⁡A+cos⁡B+cos⁡C)=4(3cos⁡Acos⁡Bcos⁡C)−3(0)=12cos⁡Acos⁡Bcos⁡C

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