single way in the network here we showed it for just W 1 and W 2 which is every single way in this network but if you have more you can just repeat it again keep applying the chain rule from output to input to compute this ok and that's the back prop algorithm in theory very simple it's just an application of the chain rule in essence but now let's touch on some of the insights from training and how you can use the back prop algorithm to train these networks in practice optimization of neural networks is incredibly tough in practice