(1)cosx≠0,则x≠kπ+π/2; k是整数;
（2）f(x)=(1-sin2x)/cosx
=[(sinx)^2+(cosx)^2 -2·sinx·cosx]/cosx
=(sinx-cosx)^2/cosx
α为第四项限角,tanα=-4/3,则sinα=-4/5,cosα=3/5
∴f(x)=(sinx-cosx)^2/cosx =49/15