[letlambda]

>Am I missing something fundamental here? Yeah, these aren't magic formula, they are just examples.

>In the first example, the method compute_error_for_line_given_points is called with values 1, 2, [[3,6],[6,9],[12,18]]. Where did those values come from?

It's an example. The first two arguments define a line y = 2x + 1, the pairs are (x,y) points being used to compute the error.

"To play with this, let’s assume that the error function is Error=x^5−2x^3−2"

This is just an example of a function used as exposition to talk about derivatives.

It isn't even an error function though. An error function has to be a function of at least two variables.