y=√[2sin^2(x)+3cosx-3]
=√[2(1-cos^2(x))+3cosx-3]
=√[-2cos^2(x)+3cosx-1]
=√[-2[cosx-3/4]^2+1/8]
又
-2cos^2(x)+3cosx-1>=0
2cos^2(x)-3cosx+1