lim(x→0)[1-cos(1-cos2x)]/x^4
=lim(x→0)1/2*(1-cos2x)^2/x^4(等价无穷小代换(1-cosx~x^2/2))
=lim(x→0)1/2*1/2*(2x)^2/x^4(等价无穷小代换(1-cosx~x^2/2))
=1