# neuralnetworksanddeeplearning.com chapter 5 problems

For http://neuralnetworksanddeeplearning.com/chap5.html , could anyone suggest:

1) how to approach the derivation of expression (123) ?

2) what constitutes value ~ 0.45 ?

3) why the need of taylor series when we can observe the identity property without any maths proof (input == output) ?