Focus of an inconic

1) Let P be the perspector of the conic and Pa, Pb, Pc its contact points with the sides.
2) Draw the axis e1, e2 of the conic (construction c58).
3) The axis e1 cuts BC at M and the perpendicular from Pa to BC at N.
4) Draw the circle with center the center of the conic and orthogonal to the circle with diameter MN.
5) This circle and the axis e1 intersect at a focus.