Find the value of the given fraction: 1 − ( 2 4 5 ) tan 2 1 ° + ( 4 4 5 ) tan 4 1 ° − ⋯ + ( 4 4 4 5 ) tan 4 4 1 ° ( 1 4 5 ) tan 1 ° − ( 3 4 5 ) tan 3 1 ° + ( 5 4 5 ) tan 5 1 ° − ⋯ + ( 4 5 4 5 ) tan 4 5 1 °
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Just saying, I don't think this is geometry. More like trigonometry.
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The given quotient can be written as:
Q = ∑ n = 0 2 2 ( − 1 ) n ( 2 n 4 5 ) tan 2 n 1 ∘ ∑ n = 0 2 2 ( − 1 ) n ( 2 n + 1 4 5 ) tan 2 n + 1 1 ∘ = ℜ ( ∑ n = 0 4 5 ( n 4 5 ) i tan n 1 ∘ ) ℑ ( ∑ n = 0 4 5 ( n 4 5 ) i tan n 1 ∘ ) = ℜ ( ( 1 + i tan 1 ∘ ) 4 5 ) ℑ ( ( 1 + i tan 1 ∘ ) 4 5 ) = ℜ ( sec 4 5 1 ∘ e i 4 5 ∘ ) ℑ ( sec 4 5 1 ∘ e i 4 5 ∘ ) = ℜ ( e i 4 5 ∘ ) ℑ ( e i 4 5 ∘ ) = ℜ ( 2 1 + i ) ℑ ( 2 1 + i ) = 2 1 2 1 = 1 where i = − 1 denotes the imaginary unit, and ℜ ( ⋅ ) and ℑ ( ⋅ ) , the real and imaginary parts respectively. By Euler’s formula: e i θ = cos θ + i sin θ