sec(x)'=(1/cos(x))'
=sin(x)/cos^2(x)
=sin(x)/cos(x) * 1/cos(x)
=tan(x) * sec(x)
secx=1/cosx=(cosx)^(-1)
所以(secx)'=-1*(cosx)^(-2)*(cosx)'
=-1/cos²x*(-sinx)
=sinx/cos²x
=sinx/cosx*1/cosx
=tanxsecx
sec(x)'=[1/cos(x)]'=sin(x)/cos(x)^2=1/cos(x).sin(x)/cos(x)=sec(x) tan(x)
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