Absurd Trignometric angles

Geometry Level 3

4 sin ( 10 ° ) + 3 tan ( 10 ° ) \large 4\sin(10°)+\sqrt{3}\tan(10°)

Find the value of above expression. Give your answer to 3 decimal places.


The answer is 1.

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2 solutions

Here I will use the product to sum formula, then the sum to product formula, also the fact that 2 sin ( 3 0 ) = 1 2\sin(30^\circ)=1 and 2 cos ( 3 0 ) = 3 2\cos(30^\circ)=\sqrt{3} :

4 sin ( 1 0 ) + 3 sin ( 1 0 ) cos ( 1 0 ) 4\sin(10^\circ)+\dfrac{\sqrt{3}\sin(10^\circ)}{\cos(10^\circ)}

4 sin ( 1 0 ) cos ( 1 0 ) + 3 sin ( 1 0 ) cos ( 1 0 ) \dfrac{4\sin(10^\circ)\cos(10^\circ)+\sqrt{3}\sin(10^\circ)}{\cos(10^\circ)}

2 sin ( 2 0 ) + 2 sin ( 1 0 ) cos ( 3 0 ) cos ( 1 0 ) \dfrac{2\sin(20^\circ)+2\sin(10^\circ)\cos(30^\circ)}{\cos(10^\circ)}

2 sin ( 2 0 ) + sin ( 4 0 ) sin ( 2 0 ) cos ( 1 0 ) \dfrac{2\sin(20^\circ)+\sin(40^\circ)-\sin(20^\circ)}{\cos(10^\circ)}

sin ( 2 0 ) + sin ( 4 0 ) cos ( 1 0 ) \dfrac{\sin(20^\circ)+\sin(40^\circ)}{\cos(10^\circ)}

2 sin ( 3 0 ) cos ( 1 0 ) cos ( 1 0 ) \dfrac{2\sin(30^\circ)\cos(10^\circ)}{\cos(10^\circ)}

1 \boxed{1}

Chew-Seong Cheong
Oct 28, 2015

4 sin 1 0 + 3 tan 1 0 = 4 sin 1 0 + 2 cos 3 0 sin 1 0 cos 1 0 Since cos 3 0 = 3 2 = 4 sin 1 0 + 2 ( 4 cos 3 1 0 3 cos 1 0 ) sin 1 0 cos 1 0 As cos ( 3 θ ) = 4 cos 3 θ 3 cos θ = 4 sin 1 0 + 2 sin 1 0 ( 4 cos 2 1 0 3 ) = 4 sin 1 0 + 2 sin 1 0 ( 4 4 sin 2 1 0 3 ) = 4 sin 1 0 + 2 sin 1 0 ( 1 4 sin 2 1 0 ) = 6 sin 1 0 8 sin 3 1 0 As sin ( 3 θ ) = 3 sin θ 4 sin 3 θ = 2 sin 3 0 = 1 \begin{aligned} 4 \sin 10^\circ + \color{#3D99F6}{\sqrt{3}} \tan 10^\circ & = 4 \sin 10^\circ + \color{#3D99F6}{2 \cos 30^\circ} \frac{\sin 10^\circ}{\cos 10^\circ} \quad \quad \quad \quad \quad \quad \quad \quad \quad \small \color{#3D99F6}{\text{Since } \cos 30^\circ = \frac{\sqrt{3}}{2}} \\ & = 4 \sin 10^\circ + 2 (\color{#3D99F6}{4\cos^3 10^\circ - 3\cos 10^\circ}) \frac{\sin 10^\circ}{\cos 10^\circ} \quad \quad \space \space \small \color{#3D99F6}{\text{As } \cos (3\theta) =4\cos^3 \theta - 3\cos \theta} \\ & = 4 \sin 10^\circ + 2 \sin 10^\circ (4\cos^2 10^\circ - 3) \\ & = 4 \sin 10^\circ + 2 \sin 10^\circ (4 - 4\sin^2 10^\circ - 3) \\ & = 4 \sin 10^\circ + 2 \sin 10^\circ (1 - 4\sin^2 10^\circ ) \\ & = 6 \sin 10^\circ - 8\sin^3 10^\circ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \small \color{#3D99F6}{\text{As } \sin (3\theta) =3\sin \theta - 4\sin^3 \theta} \\ & = 2\sin 30^\circ = \boxed{1} \end{aligned}

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