第三题
#include<stdio.h>
int main()
{
    int n;
    float i,k,x,a,pai = 1, PAI;
    scanf("%d",&n);
    for (i = 2; i < n; i+=2)
    {
        k = i*i;
        x = (i - 1)*(i +1);
        a= k / x;
        pai = pai*a;
    }
    PAI =pai * 2;
    printf("%f", PAI);
    getchar();
    getchar();
    return 0;
}
算不出3.141592

a = 1.0 * k / x;
试试

#include <stdio.h>
#include<math.h>
int main()
{  double pi=4*atan(1.0);printf("%.8lf",pi);
}

#include<stdio.h>
#include<math.h>
int main()
{
int sign = 1;
double pi = 0.0, n = 1.0, term = 1.0;//term表示当前项
while (fabs(term) >= 1e-6)
{
pi += term;
n += 2;
sign = -sign;
term = sign / n;
}
pi *= 4;
printf("pi=%10.8f\n", pi);
return 0;
}