EX$1$: Consider a fully connected neural network in the Figure below:

a$)$ Assuming the activation function is a Sigmoid function, write the analytical expressions for

derivatives with respect to the weights $W,$ biases $b,$ and input $x.$

b$)$ Assuming the activation function is an Identity function, $f(x)=x,$ what would be the derivatives

with respect to all the weights $W$ and biases $b?$ Comment on why this activation function is

such a bad choice for neural network learning.