So, the equation is:

Well, I first tried solving in the way i solved equations that didn't contain sqrt, dividing all the equation by the biggest power of x, in this case, dividing by

, getting

And, by limit laws i would get

, however, according to my book (which I trust more than myself) the answer is 1/6, so I tried doing in a different way:

and now getting :

, which is,

.

So, my question is, why I couldn't do it the first way? Isn't it supposed to be the same thing? I mean, clearly not but, what rule am I breaking by doing it in the first way?

Thanks in advance