lim(1/sin^2x-1/x^2)
=lim(x^2-sin^2x)/x^4
=lim(2x-2sinxcosx)/4x^3
=lim(2x-sin2x)/4x^3
=lim(2-2cos2x)/12x^2
=lim4sin2x/24x
=4/12=1/3