Question

How do you evaluate `cos150`?

Answer 1

`- sqrt3/2`

Explanation:

First method
Trig table, unit circle, and property of complementary arcs -->
`cos 150 = cos (60 + 90) = - sin 60 = - sqrt3/2`
Second method:
Use trig identity: cos (a + b) = cos a.cos b - sin a.sin b
cos (150) = cos (60 + 90) = cos 60.cos 90 - sin 60.sin 90 =
= - sin 60 = `-sqrt3/2`
Note. `cos 90^@ = 0`, and `sin 90^@ = 1`