2sin^2x - cosx -1 = 0

2(1-cos^2x)-1-cosx=0;
-1-cosx+2(-cos^2x+1)=0;
-2cos^2x+1-cosx=0;
-2cos^2x-cosx=-1;
cosx=u;
-2u^2-u+1=0;
D=9;
u=1+-3/-4;
cosx=-(1+-3)/4;
x=arccos(-(1+-3)/4);
x=pi-arccos(1+-3/4);
x=pi-arccos(1-3/4) или pi-arccos(1+3/4);
x=pi или x=pi/3