Old Dominion University: Shunichi Toida's "Properties of Set Operation"
Read this page. It is important that you become aware that sets combine under union and intersection in very much the same ways that numbers combine under addition and multiplication. For example,
Click http://web.archive.org/web/20160417054041/http://www.cs.odu.edu/~toida/nerzic/content/set/set_op_prop.html link to open resource.