Backpropagation chain rule example

Question

My question is in regards to an MIT course example.

The instructor delves into the backpropagation of this simple NN.

I have two questions.

1. Why do we seem to disregard the weights of the second layer (in blue)?
2. The red circle, should this not be $\partial y_2$ (followed by $\frac{\partial y_2}{\partial P_3}$)?

Answer 1

• Why do we seem to disregard the weights of the second layer (in blue)? -----because activation function is after that block, so differential with the o/p y is as good as with the weight.

• The red circle, should this not be ∂y2 (followed by ∂y2∂P3)? -----y2 and y1 is the o/p of the same block i.e. both must be same