Trapezoid median (midline or midsegment) definition and properties
Math Open Reference

Area of a trapezoid. Definition and formula
Math Open Reference

Trapezoid definition and properties
Math Open Reference

Definition of Thales theorem  the diameter of a circle always subtends a right angle to any point on the circle.
Math Open Reference

Definition and properties of a tetragon
Math Open Reference

Definition and properties of pythagorean triples
Math Open Reference

Pythagoras' Theorem
Math Open Reference

Definition of the triangles of a polygon, including a formula to calculate the number of them in an ngon
Math Open Reference

The sides of a polygon are defined and two formulas for finding the side length for a regular polygon.
Math Open Reference

Definition of a regular (equiangular) polygon
Math Open Reference
