I am looking over some neural network theory and came across this equation, coupled with this description (gradient descent ball-valley analogy):
''let's think about what happens when we move the ball a small amount Δv1 in the v1 direction, and a small amount Δv2 in the v2 direction. Calculus tells us that C changes as follows:''
I don't understand where this expression has come from. It looks like the product rule? Could someone either explain what it means and/or state the area of calculus this belongs to, so I can read up on it? Many thanks!