lim[(tanx/x）*（1-cosx）/x^2] (x->0)

=lim{ (sinx/x*cosx)*2[sin(x/2)]^2 /x^2}
即应用：cos2x=1-2(sinx)^2 移项替换1-cosx
=lim (sinx/xcosx) *lim (1/2)*{[sin(x/2)]/(x/2)}^2
即：构造 sinx/x的形式,利用lim sinx/x =1(x->0)
=(1*1) * [(1/2)*(1^2)]=1/2
即：lim sinx/x =1;lim 1/cosx =1(x->0)