Trig Summation

Geometry Level 5

sin 0 + sin 1 + sin 2 + sin 3 + + sin 201 5 + sin 201 6 cos 0 + cos 1 + cos 2 + cos 3 + + cos 201 5 + cos 201 6 \frac{\sin{0^{\circ}}+\sin{1^{\circ}}+\sin{2^{\circ}}+\sin{3^{\circ}}+\ldots+\sin{2015^{\circ}}+\sin{2016^{\circ}}}{\cos{0^{\circ}}+\cos{1^{\circ}}+\cos{2^{\circ}}+\cos{3^{\circ}}+\ldots+\cos{2015^{\circ}}+\cos{2016^{\circ}}}

If the above expression equals a + b a -\sqrt{a+b\sqrt{a}} for positive integers a a and b b . Find a × b a \times b .


The answer is 10.

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3 solutions

Digvijay Singh
Jul 11, 2015

We know that e i x = cos x + i sin x e^{ix}=\cos{x}+i\sin{x}

So, the summation of cos x \cos{x} and sin x \sin{x} would be the summation of real and imaginary parts of e i x e^{ix} respectively.

For the summation of sin x \sin{x} , x = 0 n sin x = x = 0 n ( e i x ) = ( x = 0 n e i x ) \displaystyle \sum_{x=0}^n \sin{x}=\displaystyle \sum_{x=0}^n \Im(e^{ix})=\Im \Bigg(\displaystyle \sum_{x=0}^n e^{ix}\Bigg) Clearly, this summation is a geometric one, so ( x = 0 n e i x ) = ( e i ( n + 1 ) 1 e i 1 ) \Im \Bigg(\displaystyle \sum_{x=0}^n e^{ix}\Bigg)=\Im \Bigg(\frac{e^{i(n+1)}-1}{e^i-1}\Bigg) = ( e i ( n + 1 ) 2 e i 2 e i ( n + 1 ) 2 e i ( n + 1 ) 2 e i 2 e i 2 ) =\Im \Bigg(\frac{e^\frac{i(n+1)}{2}}{e^\frac{i}{2}} \cdot \frac{e^\frac{i(n+1)}{2}-e^{-\frac{i(n+1)}{2}}}{e^\frac{i}{2}-e^{-\frac{i}{2}}}\Bigg) = sin ( n + 1 2 ) sin ( 1 2 ) ( e i n 2 ) =\frac{\sin(\frac{n+1}{2})}{\sin(\frac{1}{2})}\Im(e^{\frac{in}{2}}) = sin ( n + 1 2 ) sin ( n 2 ) sin ( 1 2 ) =\frac{\sin(\frac{n+1}{2}) \sin(\frac{n}{2})}{\sin(\frac{1}{2})} Doing the same for the summation of cos x \cos{x} gives x = 0 n cos x = ( x = 0 n e i x ) = sin ( n + 1 2 ) cos ( n 2 ) sin ( 1 2 ) \displaystyle \sum_{x=0}^n \cos{x}=\Re \Bigg(\displaystyle \sum_{x=0}^n e^{ix}\Bigg)=\frac{\sin(\frac{n+1}{2}) \cos(\frac{n}{2})}{\sin(\frac{1}{2})} x = 0 2016 sin x x = 0 2016 cos x = sin ( 1 2 × 201 6 ) cos ( 1 2 × 201 6 ) = tan ( 100 8 ) \frac{\displaystyle \sum_{x=0}^{2016} \sin{x}}{\displaystyle \sum_{x=0}^{2016} \cos{x}}=\frac{\sin(\frac{1}{2} \times 2016^{\circ})}{\cos(\frac{1}{2} \times 2016^{\circ})}=\tan(1008^{\circ}) = tan ( 7 2 ) = 5 2 5 =\tan(-72^\circ)=-\sqrt{5-2\sqrt{5}}

Moderator note:

Nice proof for the sum of sines and cosines of an arithmetic progression!

that very clean solution though

will jain - 5 years, 4 months ago
Aakash Khandelwal
Jul 14, 2015

above expression terminates to :

-(sin37+ sin38+sin39...........sin216)/cos37cos38......cos179)

Which further terminates to -cot(18)

Hence answer=10

We can use formula transformation sum of trigonometric functions sin and cos to product: ( sin ( 2016 ° ) + sin ( 0 ° ) ) + ( sin ( 2015 ° ) + sin ( 1 ° ) ) + + ( sin ( 1012 ° ) + sin ( 1014 ° ) ) + ( sin ( 1013 ° ) ( cos ( 2016 ° ) + cos ( 0 ° ) ) + ( cos ( 2015 ° ) + cos ( 1 ° ) ) + + ( cos ( 1012 ° ) + cos ( 1014 ° ) ) + cos ( 1013 ° ) = \frac{(\sin(2016°)+\sin(0°))+(\sin(2015°)+\sin(1°))+\dots+(\sin(1012°) + \sin(1014°)) +( \sin(1013°)}{(\cos(2016°) + \cos(0°)) + (\cos(2015°) + \cos(1°)) +\dots+ (\cos(1012°) + \cos(1014°)) + \cos(1013°)}= = ( 2 sin ( 1008 ° ) [ cos ( 1008 ° ) + cos ( 1007 ° ) + + c o s ( 1 ° ) + 1 ] ( 2 cos ( 1008 ° ) [ cos ( 1008 ° ) + cos ( 1008 ° ) + + cos ( 1 ° ) + 1 ] = tan ( 1008 ° ) =\frac{(2 \sin(1008°)[\cos(1008°) + \cos(1007°) +\dots+\ cos(1°) + 1]}{(2\cos(1008°)[\cos(1008°) + \cos(1008°) +\dots+ \cos(1°) +1]}=\tan(1008°)

did same way as yours!

will jain - 5 years, 4 months ago

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