2. The equation of the parabola is .
That means the slope of the tangent to the parabola is calculated by
3. Let be a point on the parabola. The tangent at P to the parabola has the equation:
4. The normal in P has the slope . Since the normal passes through P too it has the equation:
5. Now determine the point of intersection of the parabola and the normal.
6. Show that that means the angle .