WOW this is so cool!
(the formula)

As smile67 mentioned, a duplicate of many Kata out there either in beta or already approved.

Apart from that:

1. Needs random test cases (see my hardcoded solution)
2. [Suggestion] Please don't use Test.expect for value comparison; Test.assertEquals provides much better debug feedback which allows the user to know what has gone wrong with their code.

Hi, it's a duplicate (and not the only one;-)): https://www.codewars.com/kata/big-fibonacci-numbers-1