How can Murtagh know the Word?

Question

As those who have read Inheritance should know, Murtagh helps decide the final confrontation versus Galbatorix through use of the Word, the ancient language's true name that so rings of metamathematics.

However, it doesn't seem to explain how did Murtagh know what the Word was in the first place. When Galbatorix first used it to undo Eragon's oath to kill him, the Word never stuck in Eragon's, or others', minds. After all, it would be plenty strange if Galbatorix would let anybody on the only way to undo every spell he's ever cast and every oath of fidelity hes ever received, so I doubt Galbatorix ever let anybody else on the Word willingly.

How does Murtagh know the Word then?

Answer 1

Eragon could not learn the Word because Galbatorix set a spell in place such that he couldn't remember the word. Murtagh, being trusted, had no such restriction. Murtagh took this spell off of Eragon after the final battle, leaving him to be able to hear it.

Galbatorix didn't use the Word very frequently, so he didn't need to take such measures very often.

Answer 2

He heard the king use it before

How did Murtagh know the True Name of the Ancient Language anyway?

Christopher Paolini: The only reason Eragon and company couldn't remember the Word when Galbatorix uttered it was because of the king's magic. As soon as Murtagh was free of the king's control, he was able to recall and use the Word (Murtagh had heard the king use it before).

_{Reddit AMA 2}