5.40 Show that these facts are true. Use the definition of binomial coefficients to show that each of the following facts is true. Then restate each fact in words in terms of the number of ways that k successes can be distributed among n observations.

a. (nn)=1 for any whole number n≥1 .

b. (nn-1)=n for any whole number n≥1 .

c. (nk)=(nn-k) for any n and k with k≤n .