From this [video][1] by Andrew Ng around 5:00

![enter image description here][2]

How are $\delta_3$ and $\delta_2$ derived? 



  [1]: https://www.youtube.com/watch?v=Yd6q7Uy6Vpg&list=PLmQpd4sO8qHsPOemZ9cVe_8qirgsREeiH
  [2]: https://i.sstatic.net/2TnT1.png