If x r = cos ( 5 r π ) + i sin ( 5 r π ) , evaluate the value of:
ℜ ⎝ ⎛ r = 1 ∏ ∞ x r ⎠ ⎞ + ℑ ⎝ ⎛ r = 1 ∏ ∞ x r ⎠ ⎞ + ∣ ∣ ∣ ∣ ∣ ∣ r = 1 ∏ ∞ x r ∣ ∣ ∣ ∣ ∣ ∣
where ℜ ( z ) , ℑ ( z ) and ∣ z ∣ are the real part, imaginary part and absolute value of complex number z .
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Simple standard approach once you recognize the Geometric progression in the powers.
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x r = e n r i π
⇒ r = 1 ∏ ∞ x r n = 5 , r = 1 ∏ ∞ = x 1 x 2 x 3 x 4 ⋯ ∞ = e n 1 i π e n 2 i π e n 3 i π ⋯ ∞ = e i π ( n 1 + n 2 1 + n 3 1 + ⋯ ∞ ) = e i π n 1 × 1 − n 1 1 = e i π × n − 1 1 = cos n − 1 π + i sin n − 1 π = cos 4 π + i sin 4 π = 2 1 + 2 i
∴ R e ⎝ ⎜ ⎛ r = 1 ∏ ∞ x r ⎠ ⎟ ⎞ + I m ⎝ ⎜ ⎛ r = 1 ∏ ∞ x r ⎠ ⎟ ⎞ + ∣ ∣ ∣ ∣ ∣ ∣ ∣ r = 1 ∏ ∞ x r ∣ ∣ ∣ ∣ ∣ ∣ ∣ = 2 + 1 ≈ 2 . 4 1 4 2