cos(4x)= -sqrt(2)/2
4x = 3π/4 + 2πk
4x = - 3π/4 + 2πk = 5π/3 + 2πk
x = 3π/16 + πk/2
x = 5π/16 + πk/2
cos(4x) < -sqrt(2)/2
x € (3π/16 + πk/2 ; 5π/16 + πk/2) ; k € Z
