Now make sentences 1.Не can ski

(1-cos2x+2(cos2x+1))/2-5cosx-7=0;
1-cos2x+cos2x+1-5cosx-7=0;
-5cosx-5=0;
-5cosx=5;
cosx=-5/5;
cosx=-1;
x=pi+2pik.
объясн.
sin^2x=(1-cos2x)/2;
cos^2x=(cos2x+1)/2.