#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>

#define SIZE 7

using std::cout;
using std::cin;
using std::endl;
double x[] = { 0.0,0.8, 1, 2, 4, 6, 8};
double y[] = { 0.0,2.1,2.15,2.2,2.3,2.4,2.5};
double a,xbeg,xend,c,f;
int i;

int main()
{
double f1(double x,double a);
double f2(double x,double a);
double fun(double a);
   setlocale(0, "");
   xbeg=0.1;xend=2;
   c=0;
   for(i=0;i<SIZE;i++)
       c+=y[i];
   cout<<"Поиск а из решения уравнения половинным делением "<<endl;
   do{
   a=(xbeg+xend)/2;
   f=fun(a);
   if(f*fun(xbeg)<0)
       xend=a;
   if(f*fun(xend)<0)
       xbeg=a;
    cout<<a<<"  "<<fun(a)<<endl;
   }while(abs(f)>0.001);

   cout<<"a = "<< a <<endl;
   cout<<"Проверка "<<endl;
   for(i=0;i<SIZE;i++)
   cout<<x[i]<<" "<<a*(1-exp(-x[i]/a))<<"  "<<y[i]<<endl;
}

double f1(double x,double a)
{
return exp(-x/a);
}

double f2(double x,double a)
{
    return exp(-2*x/a);
}

double fun(double a)
{
    double s;
    s=0;
    s=s+10*a-c;

    for(i=0;i<SIZE;i++)
        s-=f1(x[i],a)*(x[i]+2*a-y[i]-y[i]*x[i]/a);

    for(i=0;i<SIZE;i++)
        s+=f2(x[i],a)*(x[i]+a);
    return s;
}