A Proof on hypergeometric distribution

irony of truth

Show directly that the set of probabilities associated with the hypergeometric distribution sum to one.

=> I am thinking that this tells me to prove that since this is a probability distribution function, it really should sum to 1. Is that what the problem asking me to do? =)

I got this given hint in the book that I should expand the identity:
(1 + 'mu')^N = (1 + 'mu')^r (1 + 'mu')^(N-r) and equate the coefficients.

Also, how should I equate the coefficients? Should I make the 'mu' arbitrarily equal to -1? What I did is that I expanded the left side of the given equation as 1 + (N C 1)'mu' + (N C 2)'mu'^2 + ... + (N C N)'mu'^N. That's why I got stucked on thinking... how about the coefficients?