Multinomial identities

As generalizations of binomial coefficients, multinomial coefficients inherit a lot of the neat tricks we learned for binomial coefficients (but with much worse notation).

MISTAKES / subtitles corrected
Around 2:02 I make the very common mistake of saying that 1+2+1 is 3. That multinomial coefficient is (3 choose 1, 2, 0); both the video and the subtitle correct this.