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)