Small Increments:
| If z=f(x,y) then d z = | ∂ z | d x + | ∂ z | d y |
| ∂ x | ∂ y |
Rates of Change:
We can express the above expression as a rate of change with respect to some extra dimension 't'.
| d z | = | ∂ z | d x | + | ∂ z | d y |
| d t | ∂ x | d t | ∂ y | d t |
Implicit functions:
Or if we use one of the existing dimensions such as 'x':
| d z | = | ∂ z | + | ∂ z | d y |
| d x | ∂ x | ∂ y | d x |
Change of variables
Or we can change to a new set of variables (u,v):
| ∂ z | = | ∂ z | ∂ x | + | ∂ z | ∂ y |
| ∂ u | ∂ x | ∂ u | ∂ y | ∂ u |
| ∂ z | = | ∂ z | ∂ x | + | ∂ z | ∂ y |
| ∂ v | ∂ x | ∂ v | ∂ y | ∂ v |

