2cos^2x+14cosx-3+3cos^2x=0 5cos^2x+14cosx-3=0 cosx=t 5t^2+14t-3=0 D=196+60=256 t=-14+16/10=1/5 t=-30/10 не подхоодит cosx=1/5 x= + - arccos1/5+2nk