QCAA Specialist Mathematics Rates of change and differential equations
15 sample questions with marking guides and sample answers
Determine the gradient of the tangent to when .
4
2
0.5
0.25
Reveal Answer
4
Incorrect. This is the derivative of the right side () with respect to , but it fails to apply implicit differentiation to the term.
2
Correct. Using implicit differentiation, , which simplifies to . Substituting gives a gradient of 2.
0.5
Incorrect. This calculates , which is the reciprocal of the gradient. The gradient of the tangent must be .
0.25
Incorrect. This is the -coordinate of the point on the curve when (since ), not the gradient of the tangent line.
This differential equation can be used to determine the current (amperes) at time (seconds) with voltage (volts) in an electric circuit containing a resistance (ohms):
where , and are positive constants and .
Assuming that there is no current in the electric circuit initially, show that the size of the current can never be greater than .
Reveal Answer
Given when
(as )
For all
So, the size of the current can never be greater than .
| Descriptor | Marks |
|---|---|
correctly uses the separation of variables method to set up indefinite integrals | 1 |
develops a general solution of the differential equation | 1 |
uses the given condition to determine expression for the constant of integration | 1 |
rearranges relationship to express as the subject of the equation | 1 |
expresses relationship as an exponential function | 1 |
considers value of over time to determine the required limit | 1 |
The radius of a cylinder decreases at a constant rate of , while maintaining a constant height of four metres.
Given that the cylinder has an initial volume of , determine the rate of change of the volume () of the cylinder after four seconds.
Reveal Answer
Given
Given
At , (given)
At ,
When :
| Descriptor | Marks |
|---|---|
correctly determines the rule for the volume of cylinder in terms of | 1 |
correctly expresses the rate of change of the radius as a mathematical expression | 1 |
determines a general expression for | 1 |
determines initial value of | 1 |
determines value of after 4 seconds | 1 |
determines value of when | 1 |
shows logical organisation, having fully attempted the question | 1 |
An object is released from rest at a height of 100 m above the ground.
The motion of the vertical descent of the object is modelled by
where is the velocity (m s) and is the displacement from the ground (m).
Determine the velocity of the object when it strikes the ground.
Reveal Answer
Given when
Determining when
Using graph facility of GDC
or
As , the negative solution is rejected
| Descriptor | Marks |
|---|---|
correctly uses separation of variables | 1 |
correctly develops the general solution of the differential equation | 1 |
correctly uses the given position of the origin | 1 |
uses the given condition to determine value for c | 1 |
substitutes the displacement at impact to form an equation in terms of v | 1 |
determines one reasonable solution of v | 1 |
shows logical organisation communicating key steps | 1 |
A curve modelled by the relation , where and , intersects the -axis at point .
Determine the equation of the tangent to the curve at point .
Reveal Answer
Given
Determining y-coordinate of A
Determining
Determining at A.
Determining equation of tangent at A
| Descriptor | Marks |
|---|---|
correctly determines y-intercept | 1 |
correctly determines | 1 |
correctly determines | 1 |
determines an expression for using a common factor | 1 |
determines a value for at A | 1 |
determines equation of the tangent at A | 1 |
A particular solution to the differential equation , where and , passes through the origin.
Determine this solution in the form . Leave your answer in simplified form.
Reveal Answer
Given when ,
As
| Descriptor | Marks |
|---|---|
Correctly separates the variables | 1 |
Applies suitable integration methods | 1 |
Determines a value for the constant of integration | 1 |
Determines an expression for a solution that does not contain logarithms | 1 |
Expresses in terms of | 1 |
Evaluates the reasonableness of the results and expresses the solution in the form of in simplified form | 1 |
The gradient of the tangent at point A on the curve is 1.36
The -coordinate of A is
0.12
0.46
0.54
1.47
Reveal Answer
0.12
This value is incorrect and does not satisfy the relationship between the gradient and the coordinates on the curve.
0.46
This incorrect value likely results from evaluating (approx ) instead of the correct relationship derived from the derivative.
0.54
Differentiating gives . Setting yields , and substituting this back into gives .
1.47
This is the -coordinate of point A (), but the question asks for the -coordinate.
Solve the differential equation , expressing as a function of , given that .
Reveal Answer
Thus . Since then .
| Descriptor | Marks |
|---|---|
Provides the correct expression for | 4 |
Performs correct integration and finds the constant of integration | 3 |
Performs correct integration of both sides | 2 |
Separates variables correctly | 1 |
None of the above | 0 |
Consider the curve given by .
The equation of the tangent to this curve at the point , where is a real constant, will have a negative gradient when
only
only
or
Reveal Answer
While this is the range of values that yield a negative gradient, it fails to account for the fact that the point must actually lie on the given curve.
only
This is only one of the two valid values for that lie on the curve and produce a negative gradient.
This inequality is incorrect for the gradient condition, and it also ignores that the point must satisfy the curve's equation.
only
This is only one of the two valid values for that lie on the curve and produce a negative gradient.
or
Substituting into the curve's equation gives , yielding . Both of these values satisfy the condition for a negative gradient, which is .
A certain population can be approximately modelled by the differential equation
where is the population in millions and is the number of years since 1 January 2025.
Given that the population on 1 January 2025 was estimated at 0.3 million, use a calculus approach to estimate the population on 1 January 2030.
Reveal Answer
Given :
From (1) and (2):
Given when
When
Using GDC:
The estimated population on 1 January 2030 is 2.2 million.
| Descriptor | Marks |
|---|---|
correctly separates the variables | 1 |
uses partial fractions | 1 |
uses suitable integration methods to determine a solution to the given differential equation | 1 |
determines constant of integration | 1 |
determines an equation in terms of whose solution represents the required population | 1 |
estimates the required population | 1 |
Determine the gradient of the tangent to the curve at the point .
0.41
0.53
1.06
8.49
Reveal Answer
0.41
This value is incorrect. It does not match the result derived from implicit differentiation.
0.53
Differentiating implicitly gives . Solving for the gradient yields . Substituting gives .
1.06
This answer results from incorrectly differentiating as instead of , leading to .
8.49
This value is incorrect. It appears to result from multiplying by () rather than dividing by .
A tank initially contains 300 grams of salt that is dissolved in 50 L of water. A solution containing 15 grams of salt per litre of water is poured into the tank at a rate of 2 L per minute and the mixture in the tank is kept well stirred. At the same time, 5 L of the mixture flows out of the tank per minute.
A differential equation representing the mass, grams, of salt in the tank at time minutes, for a non-zero volume of mixture, is
Reveal Answer
This implies the mass of salt is constant, completely ignoring both the inflow and outflow of salt from the tank.
This ignores the inflow of salt ( g/min) and incorrectly calculates the volume as instead of .
This assumes the volume of the tank is constant at L, leading to an outflow rate of , but the volume is actually decreasing by L/min.
The rate of salt entering is g/min, and the rate leaving is the outflow rate ( L/min) times the concentration .
This incorrectly calculates the volume of the mixture at time as , which only accounts for the outflow rate and ignores the L/min inflow.
Consider the relation .
Use implicit differentiation to find at the point .
Give your answer in the form , where .
Reveal Answer
Using implicit differentiation,
At the point ,
and so
| Descriptor | Marks |
|---|---|
Correctly differentiates the relation implicitly, demonstrating appropriate use of the product and chain rules (e.g. ) | 1 |
Correctly substitutes and into the differentiated equation | 1 |
Correctly evaluates to find the final answer in the required form, | 1 |
A brumby is a free-roaming wild horse found in large numbers in parts of Australia. The culling of brumbies was banned in the year 2000. At this time the estimated population of brumbies in Kosciuszko National Park was 1600.
Scientists have modelled the population, , of brumbies in Kosciuszko National Park years since the ban, by
It can be shown that the growth rate of the population of brumbies can be expressed as
.
Use the model to determine how long it will take the brumbies to increase to a number that is triple the number when the ban came into effect.
Reveal Answer
Solve when i.e.
From CAS
| Descriptor | Marks |
|---|---|
solves for correctly | 1 |
From this model, determine the estimated long run number of brumbies in Kosciuszko National Park.
Reveal Answer
Using then hence, .
Hence in the long term, the limiting population will be 18 000 brumbies.
| Descriptor | Marks |
|---|---|
considers to use | 1 |
determines the long run number of brumbies | 1 |
Determine the values of the constants and .
Reveal Answer
as this is the limiting population.
Using from CAS (when )
Hence substituting into
i.e.
| Descriptor | Marks |
|---|---|
states the value of correctly | 1 |
states the value of correctly | 1 |
provides justification for the determination of the value for | 1 |
Determine the greatest growth rate for the population of brumbies.
Reveal Answer
Greatest rate of growth will occur when (half the limiting population)
Using
Hence greatest growth rate will be 675 brumbies per year.
| Descriptor | Marks |
|---|---|
states that the maximum growth rate occurs when | 1 |
calculates the maximum growth rate correctly and states the correct units | 1 |
Consider the curve with equation .
Find the equation of the tangent to the curve at the point .
Reveal Answer
Method 1
At :
| Descriptor | Marks |
|---|---|
Differentiates implicitly to obtain or equivalent | 1 |
Rearranges the equation to make the subject | 1 |
Substitutes and to calculate the gradient | 1 |
Determines the correct equation of the tangent | 1 |