
Triangles.
Theorems and problems about triangles.

Circles.
Theorems and problems about circles.

Triangle Centers

Archimedes' Book of Lemmas

Quadrilaterals

Adams' Circle

First AjimaMalfatti
Point

Second AjimaMalfatti
Point

Apollonius' Tangency Problem for Three Circles
Illustration with animation and sound.

The
Bevan Point

Blanchet
Theorem

Butterfly Theorem. See also:
Butterfly Theorem Puzzle

Carnot's Theorem

Ceva's Theorem

Clifford's Circles Chain Theorems

Equal Incircles
Theorem

Equilic Quadrilateral.

Euler and his beautiful and
extraordinary formula that links the 5 fundamental constants e, i, Pi , 1 and 0, together!

Eyeball Theorem.
Illustration

Gergonne
Point Theorem

Heron's Formula. Key facts
and
a purely geometric
stepbystep proof.

Johnson's Theorem

Kurschak's Tile & Theorem

Langley Problem: 20° Isosceles Triangle
Adventitious angles.

Lemoine Theorem
Animated Angle

Menelaus' Theorem

Miquel's
Pentagram. Proof

Miquel's
Pentagram. Dynamic Geometry Cabri, GSP, Cinderella, C.a.R.

Monge & d'Alembert Three
Circles Theorem I 
Dynamic Geometry Requires Java applet 1.3 or higher.

Monge & d'Alembert Three Circles Theorem II  Dynamic Geometry Requires
Java applet 1.3 or higher.

Morley's Theorem

Napoleon's Theorem

Nagel
Point Theorem. See also: Nagel Point Flowchart Proof,
Nagel Point Puzzle.

Newton's Theorem: Newton's Line

NinePoint Center,
NinePoint Circle, Euler Line

Parallelogram with Squares theorem.

Pascal's Mystic Hexagram Theorem Proof

Pentagons and Pentagrams.
Menelaus and Collinearity

Platonic Solids.
Interactive animation.

Poncelet's
Theorem

Proposed Problems about
congruence of line segments, angles, and triangles.
Level: High School, SAT Prep, College geometry.

Ptolemy's Theorem.

Ptolemy's Extension
Cyclic Quadrilateral: Ratio of the Diagonals.

Sangaku Problem

Sangaku Problem 2

Sangaku Problem 3

Sawayama Thebault's
theorem

Semiperimeter and excircles of a triangle

Semiperimeter and incircle of a triangle

Semiperimeter, incircle and excircles of a triangle

Seven Circles
Theorem

Simson Line

Simson Line:
Angle between two Simson lines

Steiner's Theorem

100° Isosceles Triangle Problem

Triangles

Triangle with the bisectors
of the exterior angles. Collinearity

Triangle
& squares. 22 theorems, visual illustrations.

Triangle with Squares 0 Two squares.

Triangle with Squares 1 Two
squares.

Triangle with Squares 2 Two
squares.

Triangle with Squares 3 Three
squares.

Triangle with Squares 4 FinslerHadwiger Theorem.

Triangle with Squares 5 Two squares, median and altitude.

Triangle with Squares 6 Four squares.

Triangle Centers

Van Aubel's Theorem Quadrilateral
with Squares

Generalizing Van Aubel: Michael de Villiers' Theorems.

Varignon and
Wittenbauer parallelograms

Adjacent Angles and
Five Bisectors