In situations involving more than two variables you can use the chain rule to connect multiple rates of change into a single equation
Equations involving derivatives (ie rates of changes) are known as differential equations
These can be solved using methods of integration (see Differential Equations )
However setting up the equation from the information given can involve the chain rule and connected rates of change
Note from the examples above that you will often need to differentiate a formula to get one of the rates of change you need
Exam Tip
These problems can involve a lot of letters – be sure to keep track of what they all refer to.
Be especially sure that you are clear about which letters are variables and which are constants – these behave very differently when differentiation is involved!