c语言用下列公式算出e值，精度为10的-9次幂不许用<math.h>

Question

Answer 1

#include <stdio.h>
int main() {
    int k=1,t=1;
    double s=1,a=1; 
    while(a>=1e-9){
        k=k*t;
        a=1.0/k;
        s+=a;
        t++;
    }
    printf("%f",s);
    return 0;
}