Another sum of sines

Geometry Level 3

sin 2 ( 5 ) + sin 2 ( 1 0 ) + sin 2 ( 1 5 ) + . . . + sin 2 ( 9 0 ) = ? \sin^2 (5^\circ)+\sin^2 (10^\circ)+ \sin^2(15^\circ)+...+\sin^2(90^\circ) = \ ?


The answer is 9.5.

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2 solutions

Patrick Corn
Jan 15, 2020

There are 18 18 terms. The last one is 1. 1. The middle one, sin 2 ( 4 5 ) , \sin^2(45^\circ), is 0.5. 0.5. Let S S be the sum of the remaining 16 16 terms. Then S = ( sin 2 ( 5 ) + sin 2 ( 8 5 ) ) + ( sin 2 ( 1 0 ) + sin 2 ( 8 0 ) ) + + ( sin 2 ( 4 0 ) + sin 2 ( 5 0 ) ) = ( sin 2 ( 5 ) + cos 2 ( 5 ) ) + ( sin 2 ( 1 0 ) + cos 2 ( 1 0 ) ) + + ( sin 2 ( 4 0 ) + cos 2 ( 4 0 ) ) = 1 + 1 + + 1 = 8. \begin{aligned} S &= \left(\sin^2(5^\circ) + \sin^2(85^\circ)\right) + \left(\sin^2(10^\circ) + \sin^2(80^\circ)\right) + \cdots + \left(\sin^2(40^\circ) + \sin^2(50^\circ)\right) \\ &= \left(\sin^2(5^\circ) + \cos^2(5^\circ)\right) + \left(\sin^2(10^\circ) + \cos^2(10^\circ)\right) + \cdots + \left(\sin^2(40^\circ) + \cos^2(40^\circ)\right) \\ &= 1 + 1 + \cdots + 1 = 8. \end{aligned} So the answer is 1 + 0.5 + 8 = 9.5 . 1 + 0.5 + 8 = \fbox{9.5}.

Chew-Seong Cheong
Jan 15, 2020

S = sin 2 5 + sin 2 1 0 + sin 2 1 5 + + sin 2 8 0 + sin 2 8 5 + sin 2 9 0 = ( sin 2 5 + sin 2 8 5 ) + ( sin 2 1 0 + sin 2 8 0 ) + ( sin 2 1 5 + sin 2 7 5 ) + + ( sin 2 4 0 + sin 2 5 0 ) + sin 2 4 5 + sin 2 9 0 = ( sin 2 5 + cos 2 5 ) + ( sin 2 1 0 + cos 2 1 0 ) + ( sin 2 1 5 + cos 2 1 5 ) + + ( sin 2 4 0 + cos 2 4 0 ) + sin 2 4 5 + sin 2 9 0 = 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 2 + 1 = 9.5 \begin{aligned} S & = \sin^2 5^\circ + \sin^2 10^\circ + \sin^2 15^\circ + \cdots + \sin^2 80^\circ + \sin^2 85^\circ + \sin^2 90^\circ \\ & = (\sin^2 5^\circ + \sin^2 85^\circ) + (\sin^2 10^\circ + \sin^2 80^\circ) + (\sin^2 15^\circ + \sin^2 75^\circ) + \cdots + (\sin^2 40^\circ + \sin^2 50^\circ) + \sin^2 45^\circ + \sin^2 90^\circ \\ & = (\sin^2 5^\circ + \cos^2 5^\circ) + (\sin^2 10^\circ + \cos^2 10^\circ) + (\sin^2 15^\circ + \cos^2 15^\circ) + \cdots + (\sin^2 40^\circ + \cos^2 40^\circ) + \sin^2 45^\circ + \sin^2 90^\circ \\ & = 1+1+1+1+1+1+1+1+\frac 12 + 1 \\ & = \boxed{9.5} \end{aligned}

@Aly Ahmed , why don't you use LaTex. It is not that difficult. Like a professional use only three dots (...) will do. Not like a kid (.................) \ [ \backslash [ \sin^2 (5^\circ)+\sin^2 (10^\circ)+ \sin^2(15^\circ)+...+\sin^2(90^\circ) = \ ? \ ] \backslash ]

Chew-Seong Cheong - 1 year, 4 months ago

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