## ch-2 How the backpropagation algorithm works

discuss **how to compute** the gradient ( $\nabla C$ ) of the cost function

Heart of backpropagation

- an expression for the partial derivative ∂ C /∂ w of the cost function C with respect to any weight w ( or bias b ) in the network
- tells us
**how quickly**the cost changes when

we change the weights and biases