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Question

If Tnθ+cosnθ, prove that
i) T3t5T1=T5T7T3

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Solution

1) T3t5T1=T5T7T3
LHS sin3θ+cos3θsin5θcos5θsin θ+cos θ
=sin3θ+sin5θcos5θsin θ+cos θ
=sin3θ(1sin2θ)+cos3θ(1cos2θ)sin θ+cos θ
=sin3θ.cos3θ+cos3θ.sin2θsin θ+cos θ
=sin2θ.cos2θ
RHS sin5θ=sin7sin θ+cos5θcos7θsin3θ+cos3θ
=sin5θ(1sin2θ)+cos5θ(1cos θ)sin3θ+cos3θ
=sin5θ.cos2θ+cos5θ.sin2θ(sin3θ+cos3θ) (Now take out common terms)
=sin2θ.cos2θ =LHS proved




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