6. Binomial coefficient

- Nice work. Thanks for stickin' with us. We're at the last step of the lesson. Earlier, I promised you a powerful counting formula. Let's work together to see if we can develop that formula. First notice that 6 x 5 x 4 looks a little like a factorial except that it's missing the 3 x 2 x 1. That means we can write 6 x 5 x 4 using factorials as 6! over 3!. Because 6! equals 6 x 5 x 4 x 3! So dividing by 3! just leaves 6 x 5 x 4. That means, we can rewrite our earlier example as 6! over 3! x 3!. To generalize this for other numbers of actors, let n be the number of actors we can pick from and let k be the size of the cast. On the first pick, we have n choices. Then, on the second pick, we have n-1 choices and so on. Notice that the number being subtracted is one less than the choice number. So, on the kth choice, you have n - (k-1) choices which is n - k +1. Multiplying the choices together gives n x n - 1 through n - k + 1 which can be written as n! over (n-k)!. Now, we have to divide by k! because there are k! ways to order the k choices. So, finally, we get to, wait for it. Drum roll, please! n! over k!(n-k)! possible casts of k actors chosen from a group of n actors total. This formula is so famous that it has a special name and a special symbol to write it. It's called a binomial coefficient and mathematicians write it as n choose k equals n! divided by k! (n-k)!. It's powerful because you can use it whenever you're selecting a small number of things from a larger number of choices. With this tool, we can easily compute, say, how many casts of 4 robots I can come up with when I have, let's say, 12 different robots to choose from. There are 12 choose 4, which, if you work it out, is exactly 495. Your final challenge, should you choose to accept it, is to answer some final questions with the binomial coefficient formula and there won't be any diagrams to help you this time. And, you'll be asked to count something other than robots, like, let's say, plants, or sandwiches, or outfits.