How do you evaluate #cos150#?

1 Answer
May 25, 2016

#- 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#