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Question

Vectors a and b include an angle θ between them. If (a+b) and (ab) respectively subtend angles α and β with a, then (tanα+tanβ) is :

A
(absinθ)(a2+b2cos2θ)
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B
(2absinθ)(a2b2cos2θ)
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C
(a2sin2θ)(a2+b2cos2θ)
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D
(B2sin2θ)(a2b2cos2θ)
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Solution

The correct option is B (2absinθ)(a2b2cos2θ)
Given that the angle between a and b is θ

Since (a+b) makes an angle of α with a, hence

tan α=bsin θ(a+bcos θ)

Similarly, (ab) makes an angle of β with a, Hence

tan β=bsin θ(abcos θ)


Adding both the equation we get,

tan α+tan β=bsin θ(a+bcos θ)+bsin θ(abcos θ)

tan α+tan β=bsinθ[1(a+bcosθ)+1(abcosθ)]

tan α+tan β=bsinθ[(abcosθ+a+bcosθ)(a+bcosθ)(abcosθ)]

tan α+tan β=2absinθa²b²cos²θ



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