Math Planet
Math Planet: Algebra 1: Properties of Equalities
Provides examples and a video lesson that illustrates the properties of equalities.
Learning Wave Communications
Learning Wave: Commutative & Associative Properties of Addition
This resource provides information about commutative and associative properties of addition.
Help Algebra
Help algebra.com: Addition Principle
Beginning with a discussion of equivalent equation, this site describes the Addition Principle and how it is used to help solve algebraic equations.