# Prove Triangle Theorems

Videos, solutions, and lessons to help High School students learn how to prove
theorems about triangles.

Theorems include: a line parallel to one side of a triangle
divides the other two proportionally, and conversely; the
Pythagorean Theorem proved using triangle similarity.
Common Core: HSG-SRT.B.4

Related Topics:

Common
Core (Geometry)

Common Core
for Mathematics

## Triangle Proportionality Theorem

The Triangle Proportionality Theorem

If a line parallel to one side of a triangle intersects the other
two sides, then it divides those sides proportionally. The segment
joining midpoints of two sides of a triangle is parallel to the
third side and half the length.

Demonstration of the Triangle Proportionality Theorem

In the triangle ABC let PQ be parallel to BC, with P on AB and Q on
AC. Then |AP|/|PB| = |AQ|/|QC|.

Using the Properties of the Triangle Proportionality Theorem to
Solve for Unknown Values.

Proof: Converse of the Triangle Proportionality Theorem

Proving -- Converse of the Triangle Proportionality Theorem: If a
line divides two sides of a triangle proportionally, then it is
parallel to the third side.

## Pythagorean Theorem

Prove the Pythagorean Theorem using similar triangles

In this lesson, you will learn how to prove the Pythagorean Theorem
using similar triangles.

Pythagorean Theorem Proof Using Similarity

Proof of the Pythagorean Theorem using similarity.

Similar Triangles: Ratio of Areas.