Summation of trigonometry functions

Geometry Level 4

sin 2 0 + sin 2 1 0 + sin 2 2 0 + sin 2 3 0 + + sin 2 18 0 cos 2 0 + cos 2 1 0 + cos 2 2 0 + cos 2 3 0 + + cos 2 18 0 \large\dfrac{\sin^2{0^{\circ}}+\sin^2{10^{\circ}}+\sin^2{20^{\circ}}+\sin^2{30^{\circ}}+\ldots+\sin^2{180^{\circ}}}{\cos^2{0^{\circ}}+\cos^2{10^{\circ}}+\cos^2{20^{\circ}}+\cos^2{30^{\circ}}+\ldots+\cos^2{180^{\circ}}}

If the value of the above expression is a b \large\dfrac{a}{b} where a a and b b are coprime positive integers. Find the value of a 2 + b 2 a^2+b^2 .


The answer is 181.

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3 solutions

Mateus Gomes
Feb 11, 2016

sin 2 ( θ ) = sin 2 ( 18 0 θ ) sin 2 ( θ ) = cos 2 ( 9 0 θ ) cos 2 ( θ ) = cos 2 ( 18 0 θ ) \color{#3D99F6}{\boxed{\color{forestgreen}{\boxed{\sin^2(\theta)=\sin^2(180^{\circ}-\theta)\\ \sin^2(\theta)=\cos^2(90^{\circ}-\theta)\\ \cos^2(\theta)=\cos^2(180^{\circ}-\theta)}}}}

sin 2 0 + sin 2 1 0 + sin 2 2 0 + sin 2 3 0 + + sin 2 18 0 \large{\boxed{\sin^2{0^{\circ}}+\sin^2{10^{\circ}}+\sin^2{20^{\circ}}+\sin^2{30^{\circ}}+\ldots+\sin^2{180^{\circ}}}} 2 [ sin 2 0 + sin 2 1 0 + sin 2 2 0 + sin 2 3 0 + + sin 2 8 0 ] + sin 2 9 0 \large2[\sin^2{0^{\circ}}+\sin^2{10^{\circ}}+\sin^2{20^{\circ}}+\sin^2{30^{\circ}}+\ldots+\sin^2{80^{\circ}}]+ \sin^2{90^{\circ}} 2 [ 4 + sin 2 0 ] + sin 2 9 0 \large2[4+\sin^2{0^{\circ}}]+\sin^2{90^{\circ}} 2 [ 4 ] + sin 2 9 0 = 9 \large2[4]+\sin^2{90^{\circ}}=9 sin 2 0 + sin 2 1 0 + sin 2 2 0 + sin 2 3 0 + + sin 2 18 0 = 9 \rightarrow\sin^2{0^{\circ}}+\sin^2{10^{\circ}}+\sin^2{20^{\circ}}+\sin^2{30^{\circ}}+\ldots+\sin^2{180^{\circ}} ={\boxed{9}}

cos 2 0 + cos 2 1 0 + cos 2 2 0 + cos 2 3 0 + + cos 2 18 0 \large{\boxed{\cos^2{0^{\circ}}+\cos^2{10^{\circ}}+\cos^2{20^{\circ}}+\cos^2{30^{\circ}}+\ldots+\cos^2{180^{\circ}}}} 2 [ cos 2 0 + cos 2 1 0 + cos 2 2 0 + cos 2 3 0 + + cos 2 8 0 ] + cos 2 9 0 \large2[\cos^2{0^{\circ}}+\cos^2{10^{\circ}}+\cos^2{20^{\circ}}+\cos^2{30^{\circ}}+\ldots+\cos^2{80^{\circ}}]+ \cos^2{90^{\circ}} 2 [ 4 + cos 2 0 ] + cos 2 9 0 \large2[4+\cos^2{0^{\circ}}]+\cos^2{90^{\circ}} 2 [ 5 ] + cos 2 9 0 = 10 \large2[5]+\cos^2{90^{\circ}}=10 cos 2 0 + cos 2 1 0 + cos 2 2 0 + cos 2 3 0 + + cos 2 18 0 = 10 \rightarrow\cos^2{0^{\circ}}+\cos^2{10^{\circ}}+\cos^2{20^{\circ}}+\cos^2{30^{\circ}}+\ldots+\cos^2{180^{\circ}} ={\boxed{10}} sin 2 0 + sin 2 1 0 + sin 2 2 0 + sin 2 3 0 + + sin 2 18 0 cos 2 0 + cos 2 1 0 + cos 2 2 0 + cos 2 3 0 + + cos 2 18 0 = 9 10 = A B \large\dfrac{\sin^2{0^{\circ}}+\sin^2{10^{\circ}}+\sin^2{20^{\circ}}+\sin^2{30^{\circ}}+\ldots+\sin^2{180^{\circ}}}{\cos^2{0^{\circ}}+\cos^2{10^{\circ}}+\cos^2{20^{\circ}}+\cos^2{30^{\circ}}+\ldots+\cos^2{180^{\circ}}}=\frac{9}{10}=\frac{A}{B} A 2 + B 2 = 81 + 100 = 181 \Large\color{#3D99F6}{\boxed{\color{forestgreen}{\boxed{A^2+B^2=81+100=181}}}}

Rajath Naik
May 14, 2015

Using the basic identity sin^2@+cos^2@=1 ,and transformations... We'll get the ratio as 9/10. Note that the denominator forms Gud enough pairs to add up till 9 + 1 coz of cos^2(180) in the end of the sequence.similarly in the numerator but sin^2(180)=0..hence 9/10.. 9^2+10^2=181.

Moderator note:

Your phrasing could be improved much more. It's better to clarify how you pair up the terms.

formatting of solution can make it easy to understand to others. nice catch why numerator and denominator is differ by 1

Dhirendra Singh - 6 years, 1 month ago
I.E. Vermodov
Jan 16, 2021

Simply use sinx = [e^{ix}-e^{-ix}]/2 and the problem becomes a piece of cake.

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