
Gerald T. answered 06/06/19
M.S. in Mathematics - I Can Tutor Any Math Course
Think of "∩" as "and" and "∪" as "or".
So C∩D is "the set where z<=3 AND z<7". A bit of thought tells us this simply the set of z such that z<=3. (Draw these on a number line if that didn't make sense.) In interval notation this is (-∞,3].
C∪D is "the set where z<=3 OR z<7". Again, using a number line if necessary, this is just the set of numbers strictly less than 7. In interval notation, (-∞,7).