9+99+999+99999(N个9)是多少

问题描述:

9+99+999+99999(N个9)是多少

=(10-1)+(10^2-1)+...+(10^n-1)
=10+10^2+...+10^n-n
=(10-10^n+1)/(1-10)-n
=(10^n+1-10-9n)/9