程序代码：

#include<stdio.h>
#include<math.h>
double sq(int n)
{
    int i;
    double sum=0.0;
    for(i=7;i<=n;i++)
        if(i%3==0&&i%7==0)
            sum+=sqrt((double)i);
    return sum;
}
int main()
{
    int n;
    printf("Input n:");
    scanf("%d",&n);
    printf("The sum is %f",sq(n));
    return 0;
}