please show me step by step how to do all of 3 im finding it

please show me step by step how to do all of 3. im finding it very confusing

Solution

a) is straightforward

i,j th elementt will give j th element of the ith collection.

b) we use cantor diagonalisation method

loop over (i+j )

c) it tells countable union of countable sets is countable

d) same as c