程序代码：

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <time.h>

double calc_angle_cos(double x,double y); 

int main(void)
{ 
    const double pi=3.14159;
    double a[100],b[100];
    int i,k=0;
    double maxangle_cos=1.0,angle_cos,maxangle;
    srand((unsigned)time(0));
    printf("100 points created randomly (|x|<5,|y|<5)\n");
    for(i=0; i<100; i++)
    {
        a[i]=rand()%1000/100.0-5;
        b[i]=rand()%1000/100.0-5;
        if(b[i]==0)
        {
            i--;
            continue;
        }
        printf("(%.2lf,%.2lf)\t",a[i],b[i]);
        if(a[i]<0)
        {
            angle_cos=calc_angle_cos(a[i],b[i]);
            if(maxangle_cos>angle_cos)
            {
                maxangle_cos=angle_cos;
                k=i;
            }
        }
    }
    maxangle=acos(maxangle_cos)/pi*180;
    printf("\nMax angle (x<0) is %.1lf degree\n"
        "And created by the %dth point:(%.2lf,%.2lf)\n",maxangle,k+1,a[k],b[k]);
    return 0;
}

double calc_angle_cos(double x,double y)
{
    double angle_cos;
    double a2,b2;
    a2=x*x+y*y;
    b2=(x-1)*(x-1)+y*y;
    angle_cos=(a2+b2-1)/(2*sqrt(a2)*sqrt(b2));
    return angle_cos;
}