=4*sin(5x/2)*cosx*cos(x/2) =0

sin(5x/2) = 0
cosx = 0
cos(x/2) = 0

0≦x＜2πより
0≦5x/2＜5π
0≦x/2＜π

sin(5x/2) = 0　⇔　5x/2 = 0 , π , 2π , 3π , 4π
cosx = 0　⇔　x = π/2 , 3π/2
cos(x/2) = 0　⇔　x/2 = π/2

これら全てが解