visit: combination of math, ex_Lucas

I did not do ex_Lucas students can take a look at this: math thematic groupings "gift" solution to a problem . That question the way to the water.

There is a formula very important, but there are still people do not know?

There were a total of k kinds of n items, each have A _{. 1} , A ₂ , A _{. 3} ... A _n a different number thereof arrangement is essentially

$ \frac{k!}{a_1 ! \times a_2 ! \times a_3 ! \times ... \times a_n !}  (\sum\limits_{i=1}^n a_i =k) $

To explain thing? Explain it.