My 2 cents:

While in a "learning" perspective this would be considered "cheating" as it completely misses the (might or might not exist) lesson, it's completely irrelevant to what best practice is. Best practice is simply the most efficient and readable code, period. Showing the explicit form as the best practice teaches you to not blindly applying loops to problems all the time, just like you'd always use good external libraries and built-in objects so you wouldn't have to reimplement everything or write code in a too low-level manner. That's how programming irl works.

First, how would deriving the series math not be the best practice (even from a learning perspective)? Loops and recursion are costly, and I find a lot of my job (software engineer) is reducing inefficiencies in hastily-written code. When presented with a series, every programmer's second thought (the first being, of course, loop or recurse this) should be, "Can I math it?" This is the 6th kyu; the training wheels are coming off, and efficiency should matter.

Second, everyone should Google everything they don't know at all times. Heck, I derived the series math, but I ended up with an equation with respect to triangular(n + 1) that's less efficient than this one, and it took me 10 times as long to do it. I didn't learn anything doing that, I just applied what I already knew. That's a waste of my time. I could have spent that 10 minutes reading about why the series progresses the way it does, instead of just deriving a solution that works.

In my experience, the problem with a lot of programmers is that they will bull-headedly apply what they know (loop/recurse everything!), even if it only kind of works (or works poorly), instead of finding a different, better approach (Google the series). If someone decides to copy-paste the solution without learning anything, well, that'll only get them so far, even in CodeWars. Additionally, if someone is on CodeWars, they've indicated a desire to better themselves, so I'm willing to give them the benefit of the doubt and assume most people aren't just copy-pasting.

But that's exactly what a best practive is — chosing the best-suited answer from a number of different ones.
In my real code I would like to have this one, not any other one. Thus, this is the best practice.

(And yes, googling for the formula instead of taking a straightforward looping path is a good habit to have, IMO.)