x->0
sin2x = 2x -(1/6)(2x)^3 +o(x^3) = 2x- (4/3)x^3 +o(x^3)
x-(1/2)sin2x = (2/3)x^3 +o(x^3)
lim(x->0) ( x- sinx.cosx) /x^3
=lim(x->0) [ x- (1/2)sin2x ] /x^3
=lim(x->0) (2/3)x^3 /x^3
=2/3
