Draw two perpendicular diameters of the given circle (I.11). Through
each endpoint draw the tangent to the circle (III.16). Each tangent
is perpendicular to its diameter (III.18); the four tangents thus form
a quadrilateral with all sides parallel and all angles right (I.28,
I.29). Equal radii combined with I.34 force the sides equal, hence a
square circumscribes the circle.