How do you evaluate cos(2π/3)?

1 Answer
Nov 5, 2015

Answer:

#-1/2#

Explanation:

This is easy if you know basic angles with radians and the basic values of cosine.

#pi = 180# degrees
#2pi = 180 * 2# degrees #= 360# degrees
#(2pi)/3 = (360 degrees)/3 = 120# degrees

If you know the unit circle, you know that #cos(120)# is the opposite of #cos(60)# (both in degrees). This is because they are in different quadrants.

Since #cos(60) = 1/2#
#cos(120) = -1/2#

PM me if you don't know how the unit circle works :-).
Hope this helped.