shortest solution with lowest effort of computation. Should be on top.

This Kata took over a year for me, simply because I missed reading the documentation on a function clear enough... ;-D Came back today and finally the solution popped into my face!

This solution works only on the three example cases in the exact sequence, where 4, 11, 1 are expected. After that it will always return undefined.
However I am rather interested how you got to submit this solution :-)

Basic math question on the example: Isn't ((0.5 + 9) - 8)^2 supposed to be 2.25 and not 6.25? I mean neither the naive approach nor binomic formula result in 6.25 for (9.5 - 8)². Am I missing something here, that is about correct setting of braces or something?