y=2cos²x-3cosx+1
=2(cosx-3/4)²-9/8+1
=2(cosx-3/4)²-1/8
cosx=3/4时,最小值为-1/8
y=cos²x-3cosx+2
=(cosx-3/2)²-1/4
cosx=1时,(cosx-3/2)²最小1/4
此时y最小0
f(x)=lg[(1+sinx)/cosx]
f(-x)=lg[(1+sin-x)/cos-x]
=lg[(1-sinx)/cosx]
=lg[cos²x/cosx(1+sinx)]
=lg[cosx/(1+sinx)]
=-f(x)