=lim((1/cos^2x)-1)/(2xtanx+(x^2/cos^2x))
=lim(1-cos^2x)/(2xsinxcosx+x^2)
=lim(2sinxcosx)/(2sinxcosx+2x(cos^2x-sin^2x)+2x)
=limsin2x/(sin2x+2xcos2x+2x)
=lim2cos2x/(2cos2x+2cos2x-4xsin2x+2)
=2/6
=1/3