lim（sec5x-tanx）=lim (1/cos5x - sinx/cosx)=lim (cosx - sinxcos5x)/cos5xcosx令t=π/2 - x,则t→0原式=lim (sint-costsin5t)/sin5tsint由等价无穷小知sin5t∽5tsint∽t代换,得到原式=lim (sint - costsin5t)/5t&...