Demystifying Neural Networks
http://scs.ryerson.ca/~aharley/neural-networks/
Gradients
https://www.khanacademy.org/math/multivariable-calculus/multivariable-derivatives/gradient-and-directional-derivatives/v/gradient
Graphical Understanding of Partial Derivatives
https://www.khanacademy.org/math/multivariable-calculus/multivariable-derivatives/partial-derivatives/v/partial-derivatives-and-graphs