parabola function helps for you !!!
k = x1 * x2 * (x2 - x1);
a = y2 * x1 -  y1 * x2;               // a = (y2 * x1 -  y1 * x2) / k;
b = y1 * x2 * (x2 - x1) - a * x1;     // b = (y1 * x2 * (x2 - x1) - a * x1) / k

if(y[i] * k == a * x[i] * x[i] + b * x[i])   delete index i ...