NUmeric : Bisection code

Sample equation for Bisection

 

F(x)=7.50x⁴+8.5x³-11.5x²+12x¹-55

//lab : bisection
#include<stdio.h>
#include<math.h>
#include<iostream>
using namespace std;

int main()
{
    float xi,xu,xr,Es,a[40],old=0,Ea;
    int n,K=0;
    cout<< "HIghest power in the equation : " ;
    cin>>n;
    //.........................................
    printf("Enter Xi =");
    scanf("%f",&xi);
    printf("Enter Xu =");
    scanf("%f",&xu);
    printf("Enter Es =");
    scanf("%f",&Es);
    //..........................................
    printf("\n");
    printf("enter coefficient :\n");
    for (int i=n; i>=0; i--)
    {
        printf("X^%d =",i);
        scanf("%f",&a[i]);
    }
    //...........................................
    while(1)
    {
        xr=(xi+xu)/2;
        float fxi=0,fxu=0,fxr=0;
//
        for (int i=n; i>=0; i--)
        {
            fxi=fxi+a[i]*pow(xi,i);
            fxu=fxu+a[i]*pow(xu,i);
            fxr=fxr+a[i]*pow(xr,i);
        }
   printf("\n\n\nSTEP no: %d\n",K+1);
        printf("Xi   =  %.2f",xi);
        printf("\nfxi  =  %.2f",fxi);
//
        printf("\nXu   =  %.2f",xu);
        printf("\nfxu  =  %.2f",fxu);
//
        printf("\nXr   =  %.2f",xr);
        printf("\nfxr  =  %.2f",fxr);
//
        Ea=((xr-old)/xr)*100;

        old = xr;

        //cout <<" now =" <<old;
        if(Ea<0)
        {
            Ea=Ea*(-1);
        }
        printf("\nEa= %f",Ea);

        if(fxr == 0)
        {
            break ;
        }
        if((fxr*fxu)<0)
        {
            xi=xr;
            printf("\n\n xI = %f\n\n",xi);
        }
        else if ((fxr*fxi)<0)
        {
            xu=xr;
            printf("\n\n Xu =%f\n\n",xu);
        }
        if(Ea<Es)
        {
            break;
        }
        K++;
    }

   return 0;
}