Section 2: Problem 3 Solution
Working problems is a crucial part of learning mathematics. No one can learn topology merely by poring over the definitions, theorems, and examples that are worked out in the text. One must work part of it out for oneself. To provide that opportunity is the purpose of the exercises.
James R. Munkres
Show that (b), (c), (f), and (g) of Exercise 2 hold for arbitrary unions and intersections.
The proofs are almost identical to those in 2 (b), (c), (f) and (g). One just needs to replace pairwise unions and intersection (as well as some wording) with arbitrary unions and intersections.