Mathigon: Geometry: Triangles and Trigonometry: Introduction

As the fourth part of a series on the Pythagorean Theorem, Sal continues an exploration of the ways to calculate the length of sides of a right triangle using algebra. He also used this theorem to solve problems involving a right...

This lesson focuses on the properties of triangles, a closed shape that has three sides, three vertices, and and three internal angles. There are different types of triangles such as a right triangle, obtuse, and acute based on the...

This lesson focuses on the real world applications using triangles and trigonometry.

This lesson focuses on triangle congruence; it shows the ways to prove that triangles are congruent.

This lesson focuses on trigonometry including the trigonometric functions: sine, cosine, and tangent.

Pythagoras' Theorem: In any right-angled triangle, the square of the length of the hypotenuse (the side that lies opposite the right angle) is equal to the sum of the squares of the other two sides. (A+B=C)

This lesson is an introduction to transformation and symmetry by showing examples in nature and in buildings.

Don't go off on a tangent. An instructive video describes how to find the sine, cosine, and tangent ratios of an angle in a right angle. It describes how to label the sides of a right triangle and how to use mnemonics to remember the...

This lesson focuses on the characteristics of a circle including similarity, radius, circumference, and Pi. It provides formulas to determine the area of a circle, calculating Pi, and learning exercises throughout.

Don't believe everything that you hear — make them prove it! The video shows two methods to prove the Triangle Angle Sum Theorem. The following two videos apply the theorem to finding missing angle measures.