Fundamental of Machine learning

Ref:

1.  http://www.columbia.edu/itc/sipa/math/linear.html (linear function basic description)
2.  https://medium.com/human-in-a-machine-world/mae-and-rmse-which-metric-is-better-e60ac3bde13d  (RMSE and MAE description)
3.  https://towardsdatascience.com/how-to-select-the-right-evaluation-metric-for-machine-learning-models-part-1-regrression-metrics-3606e25beae0 (regreation error matric)
4.