lim tan(tanx)-sin(sinx) = lim tan(tanx)- lim sin(sinx)=lim tanx-lim sinx=lim tanx-sinx =lim sinx*(1/cos x -1)=lim 2*sinx *[sin(x/2)]^2/cosx =lim 2*sinx*[sin(x/2)]^2=lim 1/2 *x^3
故lim tan(tanx)-sin(sinx)为x的三阶无穷小