Fill in the blank below. The of sets A and B, written A U B, is the set of elements that are members of either A or B (or both). Using set-builder notation, A U B = {x : x is a member of A or x is a member of B} The of sets A and B, written A U B, is the set of elements that are members of either A or B (or both) Using set-builder notation, A U B = {x x is a member of A or x is a member of B}