#include<stdio.h>
//以下為求階層之和

/*int main()
{
int n = 1;
double a = 1, sum = 0;
while (n <21);
{
	a = a*n;
	sum = sum + a;
	n++;
}
   printf("1!+2!+...+20!=%22.15e\n",sum);  //下面那個行，這個為啥不行？？？？？？？  
   return 0;
}
*/
//試試這種

int main()
{
	double sum = 0, a = 1;
		int n;
	for (n = 1; n < 21; n++)
	{
		a = a*n;
		sum = sum + a;
	}
	printf("1!+2!+...+20!=%22.15e\n", sum);      //double 精度高 且”%a.b e“的需要型別double的引數
	return 0;
}