I got a question that states,

It is known that the number of spanning trees in Km,n is given by the function f(m,n) where

f(m,n)= n^(m-1)*m^(n-1)

Prove that the formula gives the correct number of spanning trees for K2,n.

First of all I found that f(2,n)= n*2^(n-1)

Then i tried to come up with some sort of reasoning about the number of spanning trees, and would then show that it is equivalent to the above, but i'm going round in circles really. I may be going the wrong way about it completely, but not sure, any help would be much appreciated.

Thanks.