ex1- prove of derivative sigma:
https://drive.google.com/file/d/1eeWnZBirSzvW7a6p5VflJUgRlQX9kX3l/view?usp=sharing
ex2- Implementation of delta rule by linear and sigmoidal activation function:
https://colab.research.google.com/drive/13RJWsTKDxoSy4mv9zPVSoGBjd9eJyhAW?usp=sharing