cos^2x-5sinx*cosx+2=0
sinx=V1- cos^2x
cos^2x-5V1-cos^2x *cosx+2=0
cosx=t
t^2-5V1-t^2 * t+2=0
t^2+2=5V(1-t^2)*t
t^4+4t^2+4=25(1-t^2)t^2
t^4+4t^2+4=25t^2-25t^4
26t^4-21t^2+4=0
t=1/V2
t=2/V13
cosx=V2/2
x=pi/4+2pi*k
x=-pi/4+2pi*k
x=arccos(2/V13)+2pi*k
x=arccos(-2/V13)+2pi*k
