lim(cosa/x)^(x^2)=lim exp[(x^2)ln(cosa/x)]=lim exp[ln(cosa/x)/(1/x^2)] （利用罗比达法则）=lim exp[【ln(cosa/x)】’/【(1/x^2)】’]=lim exp{[-a*(sin a/x)/(x^2*cos a/x)]/(-2/x^3)}=lim exp[(a/2)*xsina/x]=lim exp[(a/2)*(sina/x)/(1/x)](利用罗比达法则)=lim exp[(a/2)*(cosa/x)*(-a/x^2)/(-x^2)]=lim exp[(a^2/2)*(cosa/x)]=exp(a^2/2)