Sorry for the late reply. I'm a bit of an idiot so I'll just go and state everything about this one.

The topic is about Arithmetic Sequences.

a1 is the first term, An is a specific term(so if we're looking for the 9th term, then it would be a9), d is the common difference(the d refers to the fact that the difference between two successive terms yields the constant value that was added).

e.g. 1,2,3,4,5,... find the 10th term.

a1 = 1, a10 = ?, d = 1.

The formula is:

An = a1 + (n-1)(d)

a10 = 1 + (10-1)(1)

a10 = 1+9

a10 = 10.

I could easily do this if all the terms are given, or maybe even with one or two of them is missing, but if one of the missing terms has fraction(1/2x), I just can't.