direction of that event of that vector right now the magic line here is actually how do you compute that gradient and that's something I haven't told you and that's something it's not easy at all so the question is given a loss and given all of our weights in our network how do we know which way is good which way is a good place to move given all of this information and I never told you about that but that's a process called back propagation and let's talk about a very simple example of how we can actually derive back propagation using elementary calculus so we'll start with a very