变形=(2^(1/n)-2^(1/(n+1)))/(1/n)
使用洛必达法则
=ln2[2^(1/n)*-1/n^2+2^(1/(n+1))/(n+1)^2]/(-1/n^2)
分子分母同时乘以n^2
=ln2[-2^(1/n)+2^(1/(n+1))*n^2/(n+1)^2]/(-1)
=ln2[-2^(1/n)+2^(1/(n+1))]/(-1)=0
=limn*2^(1/(n+1))*(2^(1/n-1/(n+1))-1)
=limn*1*ln2/n(n+1)
=0
无穷近似值代换a^x-1~xlna