如图所示
=lim[sec²(3x)*(3x)'] /(sec²x)=lim[3sec²(3x)] /(sec²x)=lim3cos²x/cos²(3x)=lim(-6cosxsinx) /6cos3x*(-sin3x)=limsin2x/sin6x =lim2cos2x/6cos6x =1/3cosπ/cos3π =1/3
