Two similar triangles ABC and EBD. Two line segments meet at point F. Prove that ABFC and EBDF are cyclic quadrilaterals.
A quadrilateral and a triangle. What’s the angle α?
The edges of a cyclic quadrilateral extend to two intersections. O is the circle centre. The two diagonals intersect inside the circle. What is the angle α?
The blue quadrilateral contains a purple interior point which is a corner for two right angles. Reflect the purple point about the four sides of the quadrilateral. Show that the four image points are concyclic.
A present fits inside a circle, and we tape the orange ribbons to the green midpoints of the sides, choosing them perpendicular to the opposite sides at the purple points. Show that the four ribbons have a point in common.
Four circles share one point. The quadrilateral has vertices which are intersections of pairs of circles, and three of its coloured edges are tangent to the circle of the same colour. Show that the fourth edge is also tangent to its circle.
A quarter circle of radius 1, a rectangle touching the circle in an arbitrary point and a yellow quadrilateral. What is the minimal perimeter of this quadrilateral?
A quarter circle with two chords of given length. What is the area of the orange quadrilateral?
A circle with an inscribed quadrilateral. Prove that ab=cd.
A quadrilateral with connected side midpoints. What’s blue : red : yellow?