Here is my attempt to prove part (a)
To show , first let . This means . Since , there exists such that (otherwise implies that , which is a contradiction). So, . It follows that . To show there other inclusion, let . This means for some . That is, . Since . Thus, equality follows.
I believe the second equality should be . That is, not including 1.4. The method of proof is the same as above, i.e., show inclusion in both directions. First, let . This implies , in particular, . So, . To show the other direction of the inclusion, let and note that . This implies . This proves the equality.
I hope this helps a bit.