решить уравнение
-cos2x - 1=3 cos (7пи/2-x)

cos(7П/2-x)=cos(3П/2-x)=-sinx-cos2x-1=-3sinxcos2x+1=3sinx1-2sin^2x+1=3sinx2sin^2x+3sinx-2=0sinx=t-1<=t<=12t^2+3t-2=0t=-3+-√(9+16)/4=(-3+-5)/4t=1/2sinx=1/2x=(-1)^kП/6+Пk