University of California, San Diego: Edward Bender and S. Williamson's "Arithmetic, Logic and Numbers: Sets and Functions"

Read example 1 through example 4 on pages SF-3 to SF-6. It discusses proofs for sets.

Click https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2011/09/CS202-Sets-and-Functions-Edward-Benderand-S.-Gill-Williamson.pdf link to open resource.