lim(x→0) 1/x(1/sinx-1/tanx)
=lim(x→0) 1/x(tanx-sinx)/(sinxtanx)
=lim(x→0) (tanx-sinx)/x^3
=lim(x→0) tanx(1-cosx)/x^3
=lim(x→0) (x*1/2x^2)/x^3
=1/2