Although, since Andrew wants to keep the course simple and focus more on the concepts of deep learning and not the in depth mathematics behind those concepts, around 2:55, Andrew talks about those equations, including Equation 10, but also mentions I don't want to go through the detailed derivation for this...., so this equation is just provided as it is.

If it interests you, I’d encourage you research on it on your own.

thank you Mubsi, you’re absolutely right, I hadn’t see the week 4 slide. (well, I had seen it a few weeks ago, but forgot that this equation was mentionned then). thanks much