Hi, I'm working out of Dummit and Foote and I need to know if I'm proceeding correctly in a proof. Problem: for the surjective map from A to B f, prove that the relation

a ~ b iff f(a)=f(b)

is an equivalence relation whose equivalence classes are the fibers of f.

So it makes sense but I don't know if I'm necessarily 'showing it'. I say:

if f(a) = f (b)

then a= b

so (a ~ b ) -> a=b

for any f(x) in y there exists a set containing only f(x). the preimage of any of these sets defines the fiber of f over their respective elements...

Long story short, I'm running in circles and would appreciate a helping hand. Thanks!