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Question

If cosθ=cosαcosβcosαcosβ, then prove that
tan(θ/2)=±tan(α/2)cot(β/2)

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Solution

tan2θ2=1cosθ1+cosθ=(1cosαcosβ1cosαcosβ)/(1+cosαcosβ1cosαcosβ)
=(1cosα)(1+cosβ)(1+cosα)(1cosβ)
or tan2θ2=2sin2(α/2)2cos2(α/2).2cos2(β/2)2sin2(β/2)=tan2(α/2)=±tan(α/2)cot(β/2)

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