i needed this to understand as well, now it makes sense :D

Oh, now I finally see it, so trivial. As they say, "The eyes do not see what the mind does not want.". I expected something more complicated.

Well, that is obvious, but how does it work, how is it called in JS universe so I can look up this method etc.. ?

Of course it's choosing by [n] ;-)

It seems really too easy even for a complete beginner.(which I am)

I don't understand how this works, can someone please explain? According to JS operator precedence etc., we will first do this "(n + 1)", then we multiply it by "n" and then we divide by "2". But that obviously doesn't make sense so...? :-) (just a beginner here)

Exactly, my words sir. Of course it might make common sense to some, but even if I figure that out, I will still follow the given rules, which in this case are that each side has to be bigger than 0. No matter what common sense says or does not ;-)

The descriptions is very unclear, it should simply state: Can 3 inputed int create a triangle? true/false

You need to check if all sides can create a triangle.
The sum of two sides must be greater than the third.