Let A, B, and C be sets. Show that A U B is a subset of (A U B U C).

I am not sure how exactly to go about doing this. The idea I had was to show that they were equivalent by using definitions of complements, unions...etc. but they are not equal sets.

I am assuming the method of proof used for these problems is using definitions of intersection, complement, unions...etc. And set identities.

Thank you