Rates of Change (Edexcel IGCSE Further Pure Maths)
Revision Note
Written by: Roger B
Reviewed by: Dan Finlay
Approximations Using Rates of Change
How can I use rates of change to approximate changes in value?
Remember that a derivative in maths represents a rate of change
e.g. if then
When ,
That's the rate of change of with respect to when
As soon as changes away from 2, is no longer equal to 12
But it will still be close to 12 so long as is still close to 2
We can use this to approximate the change in one variable based on a change in the other variable:
That is, when changes by a small amount
the change in the value of , ,
will be approximately equal to times
This approximation is only valid when the change in is small
The smaller the change in is,
the more accurate the approximation will be
You may need to derive rates of change starting from standard geometric formulae
e.g. the volume of a sphere is
Take the derivative with respect to ,
That's the rate of change of volume with respect to radius
You may also need to use the relation
e.g. you may need to answer a question
Then
Examiner Tips and Tricks
Look out for calculus questions asking you to 'estimate' or 'approximate' the change in a quantity
The approximation is likely to be required
Remember that's only valid when is small
Worked Example
A sphere has a radius of .
The surface area of the sphere is increased by .
Using calculus, find an estimate for the increase in the radius of the sphere. Give your answer in , correct to 2 significant figures.
'Using calculus' and 'find an estimate for the increase' are hints that we should use
Here we know the change in the surface area,
We want to estimate the change in the radius,
Write down the formula for the surface area of a sphere from the exam formula sheet
Differentiate that with respect to to find
But we need for our approximation formula, so use
Substitute that into the approximation formula
We want to know the value of that when and
(Note that that is a 'small' change, , compared to the total surface area of when )
Round to 2 significant figures, as required
Connected Rates of Change
What is meant by rates of change?
A rate of change is a measure of
how a quantity is changing
with respect to another quantity
Mathematically rates of change are derivatives
could be
the rate at which the volume of a sphere changes
with respect to how its radius is changing
Context is important when interpreting positive and negative rates of change
A positive rate of change indicates an increase
e.g. the change in volume of water as a bathtub fills
A negative rate of change indicates a decrease
e.g. the change in volume of water in a leaking bucket
If a question talks about rate of increase or decrease
make sure you use the appropriate sign (+/-)
What is meant by connected rates of change?
Connected rates of change are connected by a linking variable or parameter
They are also called 'related rates of change'
Often the linking parameter is time, represented by
seconds is the standard unit for time
but a question may use other units
e.g. Water running into a large bowl
both the height and volume of water in the bowl change with time
time is the linking parameter
What are the key ideas involved with connected rates of change?
These questions usually involve the chain rule
Different letters may be used relative to the context
e.g. for volume, for area, for height, for radius
For time problems you will often use the form
where and represent other quantities in the question
Note that
Use this if you know a derivative
but you need to know the derivative instead
Also note that the chain rule can be extended to more than two terms on the right-hand side
e.g.
This lets you connect more variables using the chain rule
Remember, a derivative is not a fraction
But if you treat the derivatives on the right-hand side of the chain rule as fractions
then their common terms should 'cancel out'
to give you the derivative on the left-hand side
This is a way to check a chain rule formula
It can also help you write the formula in the first place
How do I solve problems involving connected rates of change?
Most connected rates of change questions will involve the following steps
STEP 1
Write down the rate of change given and the rate of change requiredWrite these down as derivatives
If unsure of the rates of change involved, use the units given as a clue
e.g. (or , metres per second)
This would be the rate of change of length with respect to time
The precise 'length of what' would depend on the question
STEP 2
Use chain rule to form an equation connecting these rates of change with a third rateThe third rate of change will come from a related quantity
e.g. volume, surface area, perimeter
More complicated questions may involve more than three rates of change
But those will still be able to be connected by the chain rule
STEP 3
Write down the formula for a related quantity (volume, etc)This can then be differentiated
which will provide a formula for one or more of your rates of change
STEP 4
Substitute all the known values into the chain rule equationThen solve for the value you need to know
Examiner Tips and Tricks
To determine which rate of change to use, look at the units for help
e.g. A rate of 5 cm3 per second implies volume per time
so the rate would likely be
Worked Example
A cuboid has a fixed height of 5 cm, and a square cross-section with side length of cm in the other two dimensions.
The volume of the cuboid is increasing at a fixed rate of 20 cm3 per second.
Find the rate at which the side length is increasing at the time when the side length is 3 cm.
Write down what is given, and what we need to know
Note that length and volume are both given in terms of centimetres, so we won't need to convert any units
Write down a chain rule equation connecting and
The right-hand side should 'cancel out' to be equal with the left-hand side
This means the missing derivative must be
Now we need to find
Write down the volume of a cuboid formula,
Here the height is 5, and the length and width are both
This gives a formula connecting and
Now differentiate that with respect to
That will give a formula for in terms of
But our chain rule formula needs
We can find this using
Substitute that into the chain rule formula
We know , and we want to know when
Substitute those values into the chain rule formula
Don't forget the units when giving your final answer!
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