#include <iostream>
using namespace std;

float Interpolation(float yb, float ya, float alpha) {
    float yc;

    yc = alpha * yb + (1 - alpha) * ya;
    return yc;
}

float CalculAlpha(float xa, float xb, float xc) {
    float alpha;

    alpha = (xc - xa)/(xb - xa);
    return alpha;
}

int main() {
    float xa, ya, xb, yb, xc, yc;
    float alpha;

    cout << "Saisir xa, ya, xb, yb, xc" << endl;
    cin >> xa >> ya >> xb >> yb >> xc;

    alpha = CalculAlpha(xa, xb, xc);
    yc = Interpolation(yb, ya, alpha);
    
    cout << "yc vaut " << yc << endl;
}