root.c

root.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define N 1010
#define ERROR 0.0000001

double f(int n,double x,double c[]) {
    double fx,xi;
    int i;
    fx=c[0];xi=1;
    for (i=1;i<=n;i++) {
        xi*=x;
        fx+=c[i]*xi;
    }
    return fx;
}

int main()
{
    int i , j , n,ncase,k;
    double co[10];
    double left,right,mid,f1,f2,f3;
    freopen("root.in","r",stdin);
    scanf("%d",&ncase);
    while (ncase--) {
        scanf("%d",&n);//printf("%d %d:",n,k);
        for (i=n;i>=0;i--) scanf("%lf",co+i);
        scanf("%lf %lf",&left,&right);
        f1=f(n,left,co);
        f2=f(n,right,co);
        if (f1*f2>0) {printf("NO\n");continue;}
        while (right-left>ERROR) {
            mid=(left+right)/2;
            f3=f(n,mid,co);
            if (f1*f3>0) {left=mid;f1=f3;}
            else {right=mid;f2=f3;}
        }
        //printf("%.7lf, %.7lf, %lf %lf\n",left,right,f1,f2);
        printf("%.5lf\n",left);
    }

}