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