LIM（趋近与0）(cot^2 x-1/x^2)=lim(x^2*cos^2 x-sin^2 x)/(x^2 *sin^2 x)= lim[(x^2+1)cos^2 x-1]/x^4= lim[(1/2)*(x^2+1)(1+cos2x)-1]/x^4=lim[(1/2)*(x^2+1)(1+1-(2x)^2/2!+(2x)^4/4!+O(x^4))-1]/x^4 ;将cos2x按...