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The circles on the polygon diagonals of a complete quadrilateral as diameters are coaxal. Furthermore, the orthocenters of the four triangles of a complete quadrilateral are ...

An arrangement of points with no three collinear, or of lines with no three concurrent.

The hinge theorem says that if two triangles DeltaABC and DeltaA^'B^'C^' have congruent sides AB=A^'B^' and AC=A^'C^' and ∠A>∠A^', then BC>B^'C^'.

A similarity transformation which preserves orientation, also called a homothety.

The metric ds^2=(dx^2+dy^2)/((1-x^2-y^2)^2) for the Poincaré hyperbolic disk, which is a model for hyperbolic geometry. The hyperbolic metric is invariant under conformal ...

Three concurrent homologous lines pass respectively through three fixed points on the similitude circle which are known as the invariable points.

A bistable eight-sided polyhedron discovered by Wunderlich and Schwabe (1986).

A necessary and sufficient condition for a curve to be a helix is that the ratio of curvature to torsion be constant.

The length is the longest dimension of an object.

In the depiction of a row of identical columns parallel to the plane of a perspective drawing, the outer columns should appear wider even though they are farther away.