Triangles: Theorems and Problems 35

Nine-Point Center, Nine-Point Circle, Euler Line.
Interactive illustration.
Triangle centers.

Animated illustration of the celebrated Morley's trisector theorem
Triangle. 

Three Tangent Circles Theorem Using TracenPoche Dynamic Software
Step-by-Step construction, Manipulation, and animation.

Proposed Problem 59: Right and Equilateral Triangles, Midpoints

Isogonic-Jacobi Theorem: Using TracenPoche Dynamic Geometry Software

Marion Walter's Theorem: Triangle and Hexagon areas: Using TracenPoche Dynamic Geometry Software

Orthopole of a Triangle: Using TracenPoche Dynamic Geometry Software

Sangaku Problem (An Old Japanese Theorem). Inradii, Carnot's theorem.

Proposed Problem 58: Right Triangle, Congruence, Perpendiculars.

Euclid's Elements, Book XIII, Proposition 10 One page visual illustration.

Circulo de los Nueve Puntos: Spanish version.

Mascheroni Construction with compass alone. Midpoint of a segment.

Equal Incircles Theorem: Triangles, cevians, circles.

Stewart Theorem Triangle and a cevian.

De Gua's Theorem
Right triangles.

Proposed Problem 52: Triangle, angles, incircle.

Triangle and Squares 22
 

Feuerbach Theorem
One of the most striking theorems in triangle geometry.

Feuerbach Point Facts
Triangle centers 

Feuerbach Theorem, Acute Triangle illustration
Triangle centers. 

Feuerbach Theorem, Obtuse Triangle illustration
Triangle centers. 

Feuerbach Theorem, Right Triangle illustration