Determine the limit L for a given a, and prove that it is the limit. f(x) = x^4, a = a L = a^4. I get all the way to the following point... |x-a|x+a||x^2+a^2| < ε, I do not know how to find a "bound" for the product |x+a||x^2+a^2|. Can someone lead me in the right direction? thank you.