Question

find all solutions to the equation 2cos(x) -1 =0 on the interval [-4pi,-2pi]?

Answer 1

2cos(x)-1=0
cos(x)=1/2
this is true for x=pi/3 +- 2pi
and this is true for (5/3)pi+-2pi

Answer 2

so take pi/3 and subtract 2pi and take all the values in that interval
do the same for (5/3)pi