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Geometry Level 4

Evaluate the exact value of the summation given below, hence determine the sum of the numerator and denominator of the exact value. θ = 1 8 9 sin 6 θ \sum_{\theta=1^{\circ}}^{89^{\circ}}\sin^{6}\theta

Note: This problem appeared in Euclid 2014.


The answer is 229.

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1 solution

Shaun Loong
May 30, 2014

θ = 1 8 9 sin 6 θ = θ = 1 4 4 sin 6 θ + sin 6 4 5 + θ = 4 6 8 9 sin 6 θ \sum_{\theta=1^{\circ}}^{89^{\circ}}\sin^{6}\theta=\sum_{\theta=1^{\circ}}^{44^{\circ}}\sin^{6}\theta+\sin^{6}45^{\circ}+\sum_{\theta=46^{\circ}}^{89^{\circ}}\sin^{6}\theta

Since sin θ = cos ( 9 0 θ ) \sin\theta=\cos(90^{\circ}-\theta) and sin 4 5 = 1 2 \sin45^{\circ}=\frac{1}{\sqrt{2}} , therefore: θ = 1 4 4 sin 6 θ + sin 6 4 5 + θ = 4 6 8 9 sin 6 θ = θ = 1 4 4 ( sin 6 θ + cos 6 θ ) + 1 2 3 \sum_{\theta=1^{\circ}}^{44^{\circ}}\sin^{6}\theta+\sin^{6}45^{\circ}+\sum_{\theta=46^{\circ}}^{89^{\circ}}\sin^{6}\theta=\sum_{\theta=1^{\circ}}^{44^{\circ}}(\sin^{6}\theta+\cos^{6}\theta)+\frac{1}{2^{3}} Now, we need to simplify the summation above. Let s = sin 2 θ s=\sin^{2}\theta and c = cos 2 θ c=\cos^{2}\theta . Hence, by identity, s + c = 1 s+c=1 and 4 c s = sin 2 2 θ 4cs=\sin^{2}2\theta . So, θ = 1 4 4 ( s 3 + c 3 ) = θ = 1 4 4 ( ( s + c ) ( s 2 s c + c 2 ) \sum_{\theta=1^{\circ}}^{44^{\circ}}(s^{3}+c^{3})=\sum_{\theta=1^{\circ}}^{44^{\circ}}((s+c)(s^{2}-sc+c^{2}) = θ = 1 4 4 ( 1 ) ( ( s + c ) 3 s c ) =\sum_{\theta=1^{\circ}}^{44^{\circ}}(1)((s+c)-3sc) = θ = 1 4 4 ( 1 3 s c ) = \sum_{\theta=1^{\circ}}^{44^{\circ}}(1-3sc) = θ = 1 4 4 ( 1 3 4 ( 4 s c ) ) =\sum_{\theta=1^{\circ}}^{44^{\circ}}(1-\frac{3}{4}(4sc)) = θ = 1 4 4 ( 1 3 4 sin 2 2 θ ) 44 3 4 θ = 1 4 4 sin 2 2 θ =\sum_{\theta=1^{\circ}}^{44^{\circ}}(1-\frac{3}{4}\sin^{2}2\theta) \Rightarrow 44-\frac{3}{4}\sum_{\theta=1^{\circ}}^{44^{\circ}}\sin^{2}2\theta Applying sin θ = cos ( 9 0 θ ) \sin\theta=\cos(90^{\circ}-\theta) to the summation again yields: θ = 1 4 4 sin 2 2 θ = θ = 1 2 2 sin 2 2 θ + θ = 2 3 4 4 sin 2 2 θ \sum_{\theta=1^{\circ}}^{44^{\circ}}\sin^{2}2\theta=\sum_{\theta=1^{\circ}}^{22^{\circ}}\sin^{2}2\theta+\sum_{\theta=23^{\circ}}^{44^{\circ}}\sin^{2}2\theta = θ = 1 2 2 sin 2 2 θ + θ = 1 2 2 cos 2 2 θ θ = 1 2 2 ( sin 2 2 θ + cos 2 2 θ ) = 22 =\sum_{\theta=1^{\circ}}^{22^{\circ}}\sin^{2}2\theta+\sum_{\theta=1^{\circ}}^{22^{\circ}}\cos^{2}2\theta \Rightarrow \sum_{\theta=1^{\circ}}^{22^{\circ}}(\sin^{2}2\theta+\cos^{2}2\theta)=22 With that, we can compute θ = 1 8 9 sin 6 θ \displaystyle\sum_{\theta=1^{\circ}}^{89^{\circ}}\sin^{6}\theta to be: θ = 1 8 9 sin 6 θ = 44 3 4 ( 22 ) + 1 2 3 = 221 8 \sum_{\theta=1^{\circ}}^{89^{\circ}}\sin^{6}\theta=44-\frac{3}{4}(22)+\frac{1}{2^{3}}=\frac{221}{8} The desired answer is 221 + 8 = 229 221+8=\boxed{229} .

@Calvin Lin How do I align my workings neatly? I would like to align the equal signs in the step-by-step workings :)

Shaun Loong - 7 years ago

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You can use the align or array environment in Latex. & acts as the tab alignment.

Calvin Lin Staff - 7 years ago

Awesome solution

Aamir Faisal Ansari - 7 years ago

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