"I've got it!” (6)
(Other definitions for eureka that I've seen before include "Archimedes' shout of triumph - re a uke (anag)" , "Exclamation of triump from Archimedes" , ""I've found it!" (Archimedes)" , "Cry on a great discovery" , "I've got it!" .)
(Other definitions for aha that I've seen before include "Palindromic exclamation of triumph or surprise" , "it expresses surprise" , "Cry of triumph" , "I'm surprised" , "Exclamation of triumph or surprise, a palindrome" .)