A geometry problem by Abhyudaya Apoorva

Geometry Level 2

sin 3 5 cos 5 5 + cos 3 5 sin 5 5 csc 1 0 csc 1 0 tan 8 0 tan 8 0 = ? \large \frac{\sin35^\circ \cos55^\circ + \cos35^\circ \sin55^\circ}{\csc10^\circ \csc10^\circ - \tan80^\circ \tan80^\circ} = \, ?

3.14159 3.14159 2 2 1 1 0 0 22 7 \frac{22}{7}

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2 solutions

Abhyudaya Apoorva
Dec 31, 2016

= s i n ( 90 55 ) c o s 55 + c o s ( 90 55 ) s i n 55 c o s e c 2 ( 90 80 ) t a n 2 ( 80 ) \frac{sin(90 - 55)cos55 + cos(90 - 55)sin55}{cosec^2(90-80)- tan^2(80)}
= c o s 2 ( 55 ) + s i n 2 ( 55 ) s e c 2 ( 80 ) t a n 2 ( 80 ) \frac{cos^2(55) + sin^2(55)}{sec^2(80) - tan^2(80)}
= 1 1 \frac{1}{1}
= 1

Chew-Seong Cheong
Dec 31, 2016

X = sin 3 5 cos 5 5 + cos 3 5 sin 5 5 csc 1 0 csc 1 0 tan 8 0 tan 8 0 As sin A cos B + cos A sin B = sin ( A + B ) = sin 9 0 1 sin 2 1 0 sin 2 8 0 cos 2 8 0 Note that sin ( 9 0 x ) = cos x = 1 1 cos 2 8 0 sin 2 8 0 cos 2 8 0 = 1 1 sin 2 8 0 cos 2 8 0 As 1 sin 2 x = cos 2 x = 1 cos 2 8 0 cos 2 8 0 = 1 \begin{aligned} X & = \frac {\color{#3D99F6}\sin 35^\circ \cos 55^\circ + \cos 35^\circ \sin 55^\circ}{\csc 10^\circ \csc 10^\circ - \tan 80^\circ \tan 80^\circ} & \small \color{#3D99F6} \text{As }\sin A \cos B + \cos A \sin B = \sin (A+B) \\ & = \frac {\color{#3D99F6}\sin 90^\circ}{\dfrac 1{\color{#D61F06}\sin^2 10^\circ} - \dfrac {\sin^2 80^\circ}{\cos^2 80^\circ}} & \small \color{#D61F06} \text{Note that }\sin (90^\circ - x) =\cos x \\ & = \frac {\color{#3D99F6}1}{\dfrac 1{\color{#D61F06}\cos^2 80^\circ} - \dfrac {\sin^2 80^\circ}{\cos^2 80^\circ}} \\ & = \frac 1{\dfrac {\color{#3D99F6}1-\sin^2 80^\circ}{\cos^2 80^\circ}} & \small \color{#3D99F6} \text{As }1 - \sin^2 x = \cos^2 x \\ & = \frac 1{\dfrac {\color{#3D99F6}\cos^2 80^\circ}{\cos^2 80^\circ}} = \boxed{1} \end{aligned}

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