Find ${\displaystyle \bigcup _{i=1}^{\infty }A_{i}} and \bigcup_{i=1}^{\infty} A_{i}$ if for every positive integer i,
a) Ai = {i, i + 1, i + 2, . . .}.
b) Ai = {0, i}.
c) Ai = (0, i), that is, the set of real numbers x with
0 < x < i.
d) Ai = (i,∞), that is, the set of real numbers x with
x > i.
