#include<stdio.h>void main(){float a,b,s,sum;sum=0;for(a=1;a<=20;a++){for(s=1,b=1;b<=a;b++)s=s*b;sum=sum+s;}printf("%e\n",sum);}先求出第n项阶乘，然后把前n-1项阶乘的和加上第n项。