Strategy for Modelling Functions (DP IB Applications & Interpretation (AI)): Revision Note
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Modelling with Functions
What is a mathematical model?
A mathematical model simplifies a real-world situation so it can be described using mathematics
The model can then be used to make predictions
Be aware that extrapolating (making predictions outside of the range of the data) is not considered to be accurate
Assumptions about the situation are made in order to simplify the mathematics
Models can be refined (improved) if further information is available or if the model is compared to real-world data
How do I set up the model?
The question could:
give you the equation of the model
tell you about the relationship
It might say the relationship is linear, quadratic, etc
ask you to suggest a suitable model
Use your knowledge of each model
E.g. if it is compound interest then an exponential model is the most appropriate
You may have to determine a reasonable domain
Consider real-life context
E.g. if dealing with hours in a day then
E.g. if dealing with physical quantities (such as length) then
Consider the possible ranges
If the outcome cannot be negative then you want to choose a domain which corresponds to a range with no negative values
Sketching the graph is helpful to determine a suitable domain
Which models do I need to know?
Linear
Piecewise (linear & non-linear)
Quadratic
Cubic
Exponential
Natural logarithmic
Logistic
Direct variation
Inverse variation
Sinusoidal
Examiner Tips and Tricks
You need to be familiar with the format of the different types of equations and the general shape of the graphs they produce, you need to always be thinking "does my answer seem appropriate for the given situation?"
Sketching graphs is key
Make sure that you use your GDC to plot the relevant function(s)
Sometimes you may have to play around with the zoom function or the axes to make sure that you are focused on the relevant domain
Worked Example
A cliff has a height metres above the ground. A stone is projected from the edge of the cliff and it travels through the air until it hits the ground and stops. The vertical height, in metres, of the stone above the ground
seconds after being thrown is given by the function:
.
a) State the initial value of .
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b) Determine the domain of .
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Finding Parameters
What do I do if some of the parameters are unknown?
For some models you can use your knowledge to find unknown parameters directly from the information given
For a linear model
is the rate of change, or gradient
is the initial value
For a quadratic model,
is the axis of symmetry (this is given in the formula booklet) and is the
-value of the minimum/ maximum point
is the initial value
For a cubic model,
is the initial value
For an exponential model,
is the initial value
is the horizontal asymptote, so
is a boundary of the model
For a sinusoidal model
is the amplitude
is the principal axis
is the period
A general method is to form equations by substituting in given values
You can form multiple equations and solve them simultaneously using your GDC
You could be expected to solve a system of up to three simultaneous equations of three unknowns
This method works for all models
The initial value is the value of the function when
(or the independent variable) is 0
This is often one of the parameters in the equation of the model
Examiner Tips and Tricks
It can save you time in exams to know the properties of functions listed above that allow you to find parameters directly from the information given
Worked Example
The temperature, , of a cup of coffee is monitored. Initially the temperature is 80°C and 5 minutes later it is 40°C. It is suggested that the temperature follows the model:
where is the time, in minutes, after the coffee has been made.
a) State the value of .
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b) Find the value of .
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