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Question

The solution of
3sinθsin3θ1+cosθ+3cosθ+cos3θ1sinθ=42cos(θ+π4)

A
nπ
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B
nπ+π12
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C
nπ±π2
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D
2nπ
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Solution

The correct option is B nπ
sin3θ=3sinθ4sin3θ
cos3θ=4cos3θ3cosθ
4sin3θ1+cosθ+4cos3θ1sinθ
4sinθ(1cos2θ)1+cosθ+4cosθ(1sin2θ)1sinθ
4sinθ(1cosθ)+4cosθ(1+sinθ)
4(sinθ+cosθ)
42(cos(θΠ4))
Equating with RHS, we get
θ=nπ

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