What is the difference between the fourth and third terms of the seque

Hey guys, slightly confused with the wording on this one. Although its a pretty easy question computationally, I was a bit confused about what each "term" means.

E.g. Why isn't the first term not defined by a0, where you have 1-0 = 0 = a0

and then a1 = 2, a2=5, etc..

Why are we not including the possibility that n=0, and this is the first term?

Because had we done this, the answer would be such that the difference between the 4th term (in the sequence including a0, where a0 is the starting point) and the third term is 18-5 = 13.

Please could someone highlight where my logic is incorrect, because I had actually started this question off by computing a0 and saying that a0 is the first term in the sequence, which made the 4th term 18 and the 3rd term 5 (instead of 65 and 18 respectively if we had NOT included a0 as the first term and instead started with a1 as the first term, which the correct answer does).

Hi PVOG,
I agree with you. Note that this is not an official GMAC question, so there's definitely a chance that it could be worded better. For a question like this, I'd always expect GMAC to tell you something like sequence n is defined for all positive integers. FWIW, if you're going to notice an omission like you did, I think it's also worth pointing out that there would be no way to know that 0 is the starting point...what if the sequence is defined for all prime numbers? Or for all negative integers from -21 to -11? You're right that they need to define the domain, so why assume that 0 is the first number in that domain, either?