Question #20b1a

1 Answer
Dec 8, 2017

#[(2pi)/3, (4pi)/3]#

Explanation:

#cos x <= -1/2#
Trig table gives -->
#cos x = - 1/2 # --> #x = +- (2pi)/3#, or
#x = (2pi)/3 and x = (4pi)/3#
On the unit circle,
#cos x < - 1/2# when x varies between (2pi)/3 and (4pi)/3.
#(2pi)/3 <= x <= (4pi)/3#
Answer by closed interval: #[(2pi)/3), (4pi)/3]#
The 2 end points #(2pi)/3# and #(4pi)/3# are included in the solution set.