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Question

The absolute value of tanπ16+tan5π16+tan9π16+tan13π16 is

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Solution

Let θ=π168θ=π2
tanπ16+tan5π16+tan9π16+tan13π16=tanθ+tan5θ+tan9θ+tan13θ=tanθ+tan5θ+tan(8θ+θ)+tan(8θ+5θ)=tanθ+tan5θ+tan(π2+θ)+tan(π2+5θ)=tanθ+tan5θcotθcot5θ=tanθcotθ+tan5θcot5θ(cotθtanθ=2cot2θ)=2cot2θ2cot10θ=2[cot2θ+cot(8θ+2θ)]=2[cot2θ+cot(π2+2θ)]=2[cot2θtan2θ]=4cot4θ=4cotπ4=4

Therefore, the absolute value of the expression is 4.

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