sin²x=(1-cos2x)/2
cos²x=(cos2x+1)/2
(cosx)^4=(cos²x)²=(cos2x+1)²/4
所以f(x)
=[6(cosx)^4＋5sin²x－4]／cos2x
=(3(cos2x+1)²/2+(5(1-cos2x)/2-4)/cos2x
=[(3/2)(cos²2x+2cos2x+1)-(5/2)cos2x-(3/2)]/cos2x
=[(3/2)cos²2x+(1/2)cos2x]/cos2x
=(3/2)cos2x+(1/2)
-1=