Fraction in a fraction...

Geometry Level 3

1 sin 1 0 + 3 cos 1 0 cos 2 0 = ? \dfrac{\dfrac{1}{\sin 10^\circ} + \dfrac{\sqrt{3}} {\cos 10^\circ}} {\cos 20^\circ} = ?

No calculators please!!!


The answer is 8.

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2 solutions

Noel Lo
Apr 24, 2015

1 s i n 10 + 3 c o s 10 c o s 20 = c o s 10 + 3 s i n 10 s i n 10 c o s 10 c o s 20 = 2 ( 1 2 c o s 10 + 3 2 s i n 10 ) s i n 10 c o s 10 c o s 20 \frac{\frac{1}{sin 10} + \frac{\sqrt{3}}{cos 10}}{cos 20} = \frac{\frac{cos 10 + \sqrt{3} sin 10}{sin 10 cos 10}}{cos 20} = \frac{2(\frac{1}{2} cos 10 + \frac{\sqrt{3}}{2} sin 10)}{sin 10 cos 10 cos 20}

Using special angles, we have 2 ( s i n 30 c o s 10 + c o s 30 s i n 10 ) s i n 10 c o s 10 c o s 20 = 2 s i n ( 30 + 10 ) s i n 10 c o s 10 c o s 20 = 2 s i n 40 s i n 10 c o s 10 c o s 20 \frac{2(sin 30 cos 10 + cos 30 sin 10)}{sin 10 cos 10 cos 20} = \frac{2sin (30+10)}{sin 10 cos 10 cos 20} = \frac{2 sin 40}{sin 10 cos 10 cos 20} .

Applying double angle formula twice, 2 s i n 40 s i n 10 c o s 10 c o s 20 = 4 s i n 40 2 s i n 10 c o s 10 c o s 20 = 4 s i n 40 s i n 20 c o s 20 = 8 s i n 40 2 s i n 20 c o s 20 = 8 s i n 40 s i n 40 = 8 \frac{2 sin 40}{sin 10 cos 10 cos 20} = \frac{4 sin 40}{2 sin 10 cos 10 cos 20} = \frac{4 sin 40}{sin 20 cos 20} = \frac{8 sin 40}{2 sin 20 cos 20} = \frac{8 sin 40}{sin 40} = \boxed{8} .

Daniel Ferreira
May 3, 2015

1 sin 1 0 o + 3 cos 1 0 o cos 2 0 o = cos 1 0 o + 3 sin 1 0 o cos 1 0 o sin 1 0 o 1 cos 2 0 o = 2 ( 1 2 cos 1 0 o + 3 2 sin 1 0 o ) 1 2 ( 2 cos 1 0 o sin 1 0 o ) 1 cos 2 0 o = 4 ( cos 6 0 o cos 1 0 o + sin 6 0 o sin 1 0 o ) sin 2 0 o 1 cos 2 0 o = 4 cos 5 0 o sin 2 0 o cos 2 0 o = 4 cos 5 0 o 1 2 ( 2 sin 2 0 o cos 2 0 o ) = 8 cos 5 0 o sin 4 0 o = \\ \frac{\frac{1}{\sin 10^o} + \frac{\sqrt{3}}{\cos 10^o}}{\cos 20^o} = \\\\\\ \frac{\cos 10^o + \sqrt{3} \cdot \sin 10^o}{\cos 10^o \cdot \sin 10^o} \cdot \frac{1}{\cos 20^o}= \\\\\\ \frac{2 \cdot ( \frac{1}{2} \cdot \cos 10^o + \frac{\sqrt{3}}{2} \cdot \sin 10^o)}{\frac{1}{2} \cdot (2 \cdot \cos 10^o \cdot \sin 10^o)} \cdot \frac{1}{\cos 20^o}= \\\\\\ \frac{4 \cdot ( \cos 60^o \cdot \cos 10^o + \sin 60^o \cdot \sin 10^o)}{\sin 20^o} \cdot \frac{1}{\cos 20^o} = \\\\\\ \frac{4 \cdot \cos 50^o}{\sin 20^o \cdot \cos 20^o} = \\\\\\ \frac{4 \cdot \cos 50^o}{\frac{1}{2} \cdot (2 \cdot \sin 20^o \cdot \cos 20^o)} = \\\\\\ \frac{8 \cdot \cos 50^o}{\sin 40^o} =

Uma vez que cos 5 0 o = sin 4 0 o \cos 50^o = \sin 40^o - complementares - temos que:

8 cos 5 0 o cos 5 0 o = 8 \frac{8 \cdot \cos 50^o}{\cos 50^o} = \\\\ \boxed{\boxed{8}}

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