A derivative measures how a function’s output changes when its input changes infinitesimally. Near a point, it approximates a smooth function with a line whose slope gives the derivative: positive values indicate increase, negative values indicate decrease, and magnitude indicates rate. section 2.4.1
From derivatives to model updates
For a scalar function, the derivative points toward how the output changes. In a neural network, model weights W produce predictions and a loss value. The gradient of that loss with respect to W shows how each weight affects the loss, so updating W in the opposite direction reduces the loss. section 2.4.2 section 2.4.3
Derivatives support optimization through this progression:
- Treat the loss as a function of the model’s parameters. section 2.4.2
- Compute its gradient during the backward pass. section 2.4.3
- Move parameters opposite the gradient, scaled by the learning rate. section 2.4.3
f(x + epsilon_x) = y + a * epsilon_x
The local approximation f(x + epsilon_x) = y + a * epsilon_x makes the derivative’s slope interpretation explicit. section 2.4.1





