i agree that CP had put tenga for a reason. i don't know if u remember or not but eragon had asked oromis and brom that what about the energy itself like sun and so forth but they said that its not possible because they haven't invented the spell. i think tenga is finding a spell for that specific reason so the user of that magic will be stronger than any other beings in al. also i think tenga and angela are one of the fair folk elder than elves. and he is finding some way to change the core of the magic and help eragon to defeat galby.

It's possible that Tenga will discover how eldunari produce energy as the only explanation that Oromis can give is through the manipulation of magic thought he says that they may absorb sunlight like tree's and plants. The secret of the trees and plants may be photosynthesis.

i think tenga knows all about the eldunari's and he is searching how to help the next dragon rider. he says he is looking for the answers may be he looking at all the important question that needs answers you know like taking energy from sun and light to perform a magic, guessing true name of ancient language, guessing true name of galby's.