a: 0<=cos^2x<=1
=>0<=2*cos^2x<=2
=>1<=căn 2*cos^2x+1<=căn 3
=>-1>=-căn 2*cos^2x+1>=-căn 3
=>0>=y>=-căn 3+1
y max khi cos^2x=0
=>cosx=0
=>x=pi/2+kpi
y min khi sin^2x=0
=>sin x=0
=>x=kpi
b: 0<=sin^2x<=1
=>0<=2*sin^2x<=2
=>1<=2*sin^2x+1<=3
=>1>=y>=1/3
y=1 khi sinx=0
=>x=kpi
y=1/3 khi cosx=0
=>x=pi/2+kpi