This video tutorial [5:35] explains that if the diameter of a circle is one of the sides of a triangle inscribed in the circle then the triangle is a right triangle. This video also appears in the strand Geometry: Circles.

Demonstrates how to prove that the diagonals of a rhombus are perpendicular bisectors of each other based on knowledge of triangle congruency postulates and properties of altitudes. [4:38]

Demostrates how to prove that one way to find the area of a rhombus is by taking half the product of the diagonals. [5:07]

Demonstrates how to prove that the diagonals of a rhombus are perpendicular bisectors of each other based on knowledge of properties of parallelogram diagonals, triangle congruency, and supplementary angles. [4:04]

This video tutorial [6:56] shows a proof that the centroid is located two-thrids of the way along each median. Proof is done algebraically.

Proves that if the orthocenter and centroid are the same point in a triangle then the triangle is a right triangle. [6:25]

In this video tutorial [6:59], the instructor provides a proof for the relationship among the legs and hypotenuse of 30-60-90 triangles by breaking an equilateral triangle into two congruent triangles and using the Pythagorean threorem.

This video tutorial [15:15] provides a two-dimensional proof that the centroid is located two-thirds the way along every median of a triangle. This is a follow up to the proof in three-dimensions provided in the video, "Triangle Median...

This video tutorial [8:59] reviews the definitions of median and centroid in relation to a triangle and proves that the centroid is two-thirds the way along a median. The proof is done be drawing a triangle in three-dimensions.

Proves that the circumcenter of a right triangle is the midpoint of the hypotenuse. [5:42]

Explains using multiple proofs that a point on a perpendicular bisector is equidistant from the endpoints of the segment it bisects. Proofs use triangle congruency postulates. Lays the foundation for understanding the circumcenter of a...

This video demonstrates that the centroid divides each median into segments with a 2:1 ratio (or that the centroid is 2/3 along the median).

Video example of using deductive reasoning and writing an informal algebraic proof.

Demostrates how to take any triangle and make it become the medial triangle of a larger triangle. Also explains that the three altitudes of a triangle are concurrent. [10:01]

This video proves that the centroid of a triangle is two-thirds the way along a median.

This video tutorial demonstrates how to prove that the opposite angles of a parallelogram are congruent based on knowledge of the special angles created when parallel lines are cut by a transversal. [4:08]

Proves that the circumcenter, orthocenter, and centroid all lie on the same line, the Euler line. Proof relies on understanding of properties of similar triangles and properties of special parts of a triangle. [9:50]

Demonstrates how to prove that the diagonals of a parallelogram bisect each other based on knowledge of the special angles created when parallel lines are cut by a transversal and the triangle congruency postulates. [9:06]