Express the following sets in set-builder form.
1. {1, 4, 9,……..81}
2. {1, 5, 25, 125, 625, 3125}
3. {3, 6, 9, 12, 15, 18}

1. A = {x : x is a square of natural number and x ≤𝟖𝟏}.

2. B = {x : x = 5a, 0 ≤ a ≤ 5, a ∈ W}.

3. C = {x : x is a multiple of 3 and x < 21}