cos 0 ∘ + cos 1 ∘ + cos 2 ∘ + cos 3 ∘ + … + cos 2 0 1 5 ∘ + cos 2 0 1 6 ∘ sin 0 ∘ + sin 1 ∘ + sin 2 ∘ + sin 3 ∘ + … + sin 2 0 1 5 ∘ + sin 2 0 1 6 ∘
If the above expression equals − a + b a for positive integers a and b . Find a × b .
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Nice proof for the sum of sines and cosines of an arithmetic progression!
that very clean solution though
above expression terminates to :
We can use formula transformation sum of trigonometric functions sin and cos to product: ( cos ( 2 0 1 6 ° ) + cos ( 0 ° ) ) + ( cos ( 2 0 1 5 ° ) + cos ( 1 ° ) ) + ⋯ + ( cos ( 1 0 1 2 ° ) + cos ( 1 0 1 4 ° ) ) + cos ( 1 0 1 3 ° ) ( sin ( 2 0 1 6 ° ) + sin ( 0 ° ) ) + ( sin ( 2 0 1 5 ° ) + sin ( 1 ° ) ) + ⋯ + ( sin ( 1 0 1 2 ° ) + sin ( 1 0 1 4 ° ) ) + ( sin ( 1 0 1 3 ° ) = = ( 2 cos ( 1 0 0 8 ° ) [ cos ( 1 0 0 8 ° ) + cos ( 1 0 0 8 ° ) + ⋯ + cos ( 1 ° ) + 1 ] ( 2 sin ( 1 0 0 8 ° ) [ cos ( 1 0 0 8 ° ) + cos ( 1 0 0 7 ° ) + ⋯ + c o s ( 1 ° ) + 1 ] = tan ( 1 0 0 8 ° )
did same way as yours!
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We know that e i x = cos x + i sin x
So, the summation of cos x and sin x would be the summation of real and imaginary parts of e i x respectively.
For the summation of sin x , x = 0 ∑ n sin x = x = 0 ∑ n ℑ ( e i x ) = ℑ ( x = 0 ∑ n e i x ) Clearly, this summation is a geometric one, so ℑ ( x = 0 ∑ n e i x ) = ℑ ( e i − 1 e i ( n + 1 ) − 1 ) = ℑ ( e 2 i e 2 i ( n + 1 ) ⋅ e 2 i − e − 2 i e 2 i ( n + 1 ) − e − 2 i ( n + 1 ) ) = sin ( 2 1 ) sin ( 2 n + 1 ) ℑ ( e 2 i n ) = sin ( 2 1 ) sin ( 2 n + 1 ) sin ( 2 n ) Doing the same for the summation of cos x gives x = 0 ∑ n cos x = ℜ ( x = 0 ∑ n e i x ) = sin ( 2 1 ) sin ( 2 n + 1 ) cos ( 2 n ) x = 0 ∑ 2 0 1 6 cos x x = 0 ∑ 2 0 1 6 sin x = cos ( 2 1 × 2 0 1 6 ∘ ) sin ( 2 1 × 2 0 1 6 ∘ ) = tan ( 1 0 0 8 ∘ ) = tan ( − 7 2 ∘ ) = − 5 − 2 5