The ! is the notation for factorials. And x factorial is the product of all the natural numbers from 1 to x inclusive. So is shorthand for:
1 * 2 * 3 * ... * n * n+1
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1 * 2 * 3 * ... * n * n+1 * n+2
From this view we can see that every factor of the numerator will cancel with all but the last factor of the denominator.
So