sin 2 7 π sin 2 7 2 π + sin 2 7 2 π sin 2 7 4 π + sin 2 7 4 π sin 2 7 π = ?
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@Chew-Seong Cheong Sir , can you please explain me the third last step?
Sir , not that part .
The last third = .
How you converted that 2 n + 1 ?
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See explanation.
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S = sin 2 7 π sin 2 7 2 π + sin 2 7 2 π sin 2 7 4 π + sin 2 7 4 π sin 2 7 π = sin 2 7 π sin 2 7 2 π + sin 2 7 2 π sin 2 7 4 π + sin 2 7 4 π ( − sin 7 8 π ) 2 = n = 0 ∑ 2 sin 2 7 2 n π sin 2 7 2 n + 1 π = n = 0 ∑ 2 sin 2 7 2 n π 4 sin 2 7 2 n π cos 2 7 2 n π = n = 0 ∑ 2 4 cos 2 7 2 n π = 2 n = 0 ∑ 2 ( cos 7 2 n + 1 π + 1 ) = 2 ( cos 7 2 π + cos 7 4 π + cos 7 8 π ) + 6 = − 2 ( cos 7 5 π + cos 7 3 π + cos 7 π ) + 6 = − 2 ( 2 1 ) + 6 = 5 Note that: cos 2 x = 2 1 ( cos 2 x + 1 ) Note that: cos ( π − x ) = − cos x See note: k = 0 ∑ n − 1 cos ( 2 n + 1 2 k + 1 π ) = 2 1
Note:
S = k = 0 ∑ n − 1 cos ( 2 n + 1 2 k + 1 π ) = ℜ { k = 0 ∑ n − 1 e 2 n + 1 2 k + 1 π i } = ℜ { e 2 n + 1 π i k = 0 ∑ n − 1 e 2 n + 1 2 k π i } = ℜ { e 2 n + 1 π i ( 1 − e 2 n + 1 2 π i 1 − e 2 n + 1 2 n π i ) } = ℜ { 1 − e 2 n + 1 2 π i e 2 n + 1 π i − e π i } = ℜ ⎩ ⎨ ⎧ ( 1 + e 2 n + 1 π i ) ( 1 − e 2 n + 1 π i ) e 2 n + 1 π i + 1 ⎭ ⎬ ⎫ = ℜ { 1 − e 2 n + 1 π i 1 } = ℜ { 1 − cos 2 n + 1 π i − i sin 2 n + 1 π i 1 } = ℜ { ( 1 − cos 2 n + 1 π i ) 2 + sin 2 2 n + 1 π i 1 − cos 2 n + 1 π i + i sin 2 n + 1 π i } = ℜ { 2 − 2 cos 2 n + 1 π i 1 − cos 2 n + 1 π i + i sin 2 n + 1 π i } = 2 1