lagrang

 #include<bits/stdc++.h>
using namespace std;
double x[100];
double y[100];
double f(int p,int y,double x1)
{
    double temp=1;
    for(int i=0;i<y;i++)
    {
        if(i!=p)
        {
             temp*=(x1-x[i]);
        }

    }
    return temp;
}
double g(int s,int n)
{
    double q=x[s],temp=1;

    for(int i=0;i<n;i++)
    {
        if(i!=s)
        {
           temp*=(q-x[i]);
        }
    }
    return temp;
}
int main()
{
    int n;
    double x1,sum=0;
    cout<<"enter the total value of x and y:";
    cin>>n;

    for(int i=0;i<n;i++)
    {    cout<<"enter the element of x:";
        cin>>x[i];
        cout<<"enter the element of y:";
        cin>>y[i];

    }
    cout<<"enter the desired value:";
    cin>>x1;
    for(int i=0;i<n;i++)
    {

    sum+=(f(i,n,x1)*y[i])/g(i,n);

    }
    cout<<sum;
}