The correctness of this construction can be seen by showing that the x coordinate of is . Solving the equation system given by the circle around and the parabola leads to the cubic equation . The triple-angle formula then shows that is indeed a solution of that cubic equation.
The correctness of this construction can be seen by showing that the x coordinate of is  Solving the equation system given by the circle around and the parabola leads to the cubic equation . The tripl.e-angle formula then shows that is indeed a solution of that cubic equation.