These are not the same. The left-hand side says that A is a subset of B; the right-hand side says that the complement of A ("all elements x which areA is contained completely in B <=> for all elements x which are not elements of A are elemets of Bnotelements of A") is a subset of B.

You can use LaTeX between math and /math tags (tags use square brackets here unlike angular brackets in HTML). There is also a button on the toolbar above the text area where one types posts; it inserts the math tags.(This place could do with a maths notation font or summit )