y=(6cos^4x+5sin²x-4）/cos2x
=(6cos^4x+5-5cos²x-4)/(2cos²x-1)
=(6cos^4x-5cos²x+1)/(2cos²x-1)
=(2cos²x-1)(3cos²x-1)/(2cos²x-1)
=3cos²x-1
=3(cos2x+1)/2-1
=(3cos2x)/2+1/2
当cos2x=1时,ymax=4
当cos2x=-1时,ymin=-1