period is from 0 to 2pi
take the deritivive then take it again
we get f''(x)=-4cos²(x)-2cos(x)+2
it is negative between 0 and pi/3 and also between 5pi/3 and 2pi
concave down in the interval
[tex][0, \frac{ \pi }{3}][/tex] U [tex][\frac{ 5\pi }{3},2\pi][/tex]