Question

My book says that:

(2n)!=1*2*3*…*n*(n+1)…*2n

I don't understand that. Can some explain it to me?

Answer 1

When you take the factorial of a whole number, you multiply all the natural numbers between itself and 1, like this:
4!=4x3x2x1
3!=3x2x1
2!=2x1
...and so on. They're taking the factorial of twice a number. If you factorial (2 times 4) it will look like this:
(2x4)!=1x2x3x4...x8

Thats all it's saying.

Answer 2

$$n!=1*2*3*...*n$$
$$(2n)!=1*2*3*…*2n$$