I don’t seem to know how to get started

[MafaldaRSF]

I have to prove that f is only injective if AUB=E.

i know f is injective if every A has a B image but I don’t quite understand how to get that across in these terms.

 

[blamocur]

I've found it helpful to spell out what it means for [imath]f[/imath] to be non-injective, then figure out how to show non-injectivity when [imath]A \cup B \neq E[/imath]