Math Medics
S.o.s. Math: First Order Linear Equations
This lesson defines a first order linear equation and shows how to solve them. Also gives information on the integrating factor.
Math Medics
S.o.s. Math: Integrating Factor Technique
This math lesson defines and shows how to solve for an integrating factor.
Math Medics
S.o.s. Math: Integrating Factor Technique: Example
This lesson provides a sample equation and solution to an integrating factor problem.
Wolfram Research
Wolfram Math World: Integrating Factor
This math site defines integrating factors and has related links.