1 , cos^2x-cosx+1
<=> ( cosx-1)^2 + cosx
-1<= cosx<= 1 => GTNN = 1 khi cos x =1
GTLN = 3 khi cos x = -1
2 cos^4x -cos2x -2cos^2x +1
<=> cos^4x - 2cos^2x+1-2cos^2x +1
<=> cos^4x-4cos^2x +2
0<=cos^2x<=1
0<=cos^4x <= 1 => 0<= cosx<= 1
=> GTNN = -1 khi cos x = 1
=> GTLN =2 khi cos x = 0