Incyclic-Circumcyclic Quadrilaterals

Extract

1. Let α, β, γ, δ be parameters of four points A, B, C, D on the in-circle x² + y² = a², the coordinates of A being (a cos α, a sin α) and so on. (Fig. 1).

The tangent at A is x cos α + y sin α - a = 0. The equation