I started doing number 4 and I’m a bit confused by it. We have that and when I did the example for I got

and when I used the formula I got that

. What’s tripping me out are the middle terms of these expressions because if what we want to show holds then

but we are not told that . I’m not sure if i’m tired and this is obvious or if is a typo and it should actually be because then the equality of the middle terms is clear.

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Servando, you’re right. It is a typo; the equation should read .

I have a related question: in part (b), should it be for rather than for ?

Sure!

Hello, I also wanted to ask something. In 4b) it says to generalize it to the multinomial case. Does it mean that we just give a formula or we also need to give a formula and prove it?

you need to state it and prove it.