First slide

Trignometry

Question

Find the value of $\cos 105 ° + \cos 75 ° .$

Easy

A

Two
B

One
C

$Zero$
D

None of the above

Solution

$\cos 105 ° + \cos 75 ° = 2 \cos (\frac{105 ° + 75 °}{2})$ $\cos (\frac{105 ° - 75 °}{2})$
$= 2 (\cos 90 °) (\cos 35 °)$
$= 0 as \cos 90 ° = 0$

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