积法则
(fg)'=f'g+fg'
y'=(secθ)'tanθ+secθ(tanθ)'
=secθtanθtanθ+secθsec^2θ
=secθ(tan^2θ+sec^2θ)
=secθ(sec^2θ-1+sec^2θ)
=secθ(2sec^2θ-1)