First slide

Trignometry

Question

Find the value of $\cos 67 ° .$

Moderate

A

1
B

$(\frac{4 \sqrt{3} - 3}{5})$
C

$(\frac{4 \sqrt{3} - 3}{10})$
D

$(\frac{\sqrt{3} - 3}{10})$

Solution

$\cos 67 ° = \cos (30 ° + 37 °)$
$= (\cos 30 °) (\cos 37 °) - (\sin 30 °) (\sin 37 °)$
$= (\frac{\sqrt{3}}{2}) (\frac{4}{5}) - (\frac{1}{2}) (\frac{3}{5})$
$= (\frac{4 \sqrt{3} - 3}{10})$

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