设y=f(secx),且f(x)的导数等于x,则dy/dx | x=π/4 等于多少

题目

设y=f(secx),且f(x)的导数等于x,则dy/dx | x=π/4 等于多少
麻烦写下具体步骤,谢谢~

答案

y=f(secx)
y'=f'(secx)*secxtanx
=sec²xtanx
所以
dy/dx | x=π/4
=sec²π/4tanπ/4
=2