How to format the first step into the final answer

[fork]

Hello, everybody,

I know how to get the first step, but I don't know how can the first step be changed to the format of the final answer, can anyone help?

Thanks.

 

[pizzasky]

How many terms are in the product "(-4)(-5)...(-4-r+1)"? How can this product be combined with (-1)^r?

Also, writing out r! as a product of consecutive integers should help.

 

[HallsofIvy]

Interesting! I was wondering how that r! in the denominator magically became 6.

My suggestion (in fact what I did) is calculate both formulas for r= 1, 2, 3, 4, 5,... and see what happens.