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Prove the following: θθθθθθtan-1[cosθ + sinθcosθ - sinθ]=π4+θ,ifθ∈(-π4,π4) - Mathematics and Statistics

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Question

Prove the following:

`tan^-1["cosθ + sinθ"/"cosθ - sinθ"] = pi/(4) + θ, if θ ∈ (- pi/4, pi/4)`

Sum

Solution

L.H.S. = `tan^-1["cosθ + sinθ"/"cosθ - sinθ"]`

= `tan^-1 [((cosθ)/(cosθ) + (sinθ)/(cosθ))/((cosθ)/(cosθ) - (sinθ)/(cosθ))]`

= `tan^-1((1 + tanθ)/(1 - tanθ))`

= `tan^-1[(tan  pi/4 + tanθ)/(1 - tan  pi/4 tan θ)]`

= `tan^-1[tan(pi/4 + θ)]`

= `pi/(4) + θ`           ...[∵ tan–1(tan θ) = θ]
= R.H.S.

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Chapter 3: Trigonometric Functions - Exercise 3.3 [Page 103]

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