cos x/2+cosx=0 ПОМОГИТЕ!!!

cosx/2+2cos^2x/2-1=0
cosx/2=t
2t^2+t-1
D=9=3^2
t1=2/4
t2=-1
cosx/2=-1
x/2=-pi/2+2pin
x=-pi+4pin, n e z
cosx/2=1/2
x/2=pi/3+2pik
x=2pi/3+4pik, k e z