First of all, adjacent two phases are positive and negative, so there must be a power of-1 If the first term is positive, it must be a power of-1 or a power of-1.

Let's start with the denominator. What is the power of x, so it must be the n power of X. ..

Molecules are in trouble:

The first item 2 = 1X2

The second item 6=2X3

Item 3 12=3X4

The fourth item 20=4X5

Namely n(n+ 1)

So, the most common answer is

Molecule: 2n power of-1 times n power of x.

Denominator: n(n+ 1)