QCAA Specialist Mathematics Rates of change and differential equations

15 sample questions with marking guides and sample answers

Q6
2025
QCAA
Paper 2
1 mark
Q6
1 mark

Determine the gradient of the tangent to y2=4xy^2 = 4x when y=1y = 1.

A

4

B

2

C

0.5

D

0.25

Reveal Answer
A

4

Incorrect. This is the derivative of the right side (4x4x) with respect to xx, but it fails to apply implicit differentiation to the y2y^2 term.

B

2

Correct Answer

Correct. Using implicit differentiation, 2ydydx=42y \frac{dy}{dx} = 4, which simplifies to dydx=2y\frac{dy}{dx} = \frac{2}{y}. Substituting y=1y = 1 gives a gradient of 2.

C

0.5

Incorrect. This calculates dxdy=y2=0.5\frac{dx}{dy} = \frac{y}{2} = 0.5, which is the reciprocal of the gradient. The gradient of the tangent must be dydx\frac{dy}{dx}.

D

0.25

Incorrect. This is the xx-coordinate of the point on the curve when y=1y = 1 (since 12=4x    x=0.251^2 = 4x \implies x = 0.25), not the gradient of the tangent line.

Q18
2021
QCAA
Paper 1
6 marks
Q18
6 marks

This differential equation can be used to determine the current II (amperes) at time tt (seconds) with voltage VV (volts) in an electric circuit containing a resistance RR (ohms):

kdIdt+RI=Vk \frac{dI}{dt} + RI = V

where kk, RR and VV are positive constants and t0t \geq 0.

Assuming that there is no current in the electric circuit initially, show that the size of the current can never be greater than VR\frac{V}{R}.

Reveal Answer

kdIdt+RI=VkdIdt=VRIk \frac{dI}{dt} + RI = V \Rightarrow k \frac{dI}{dt} = V - RI
kVRIdI=1dt\int \frac{k}{V - RI} dI = \int 1 dt
kRlnVRI=t+c-\frac{k}{R} \ln|V - RI| = t + c
Given I=0I = 0 when t=0t = 0
c=kRln(V)c = -\frac{k}{R} \ln(V) (as V>0V > 0)
kRlnVRI=tkRln(V)-\frac{k}{R} \ln|V - RI| = t - \frac{k}{R} \ln(V)
lnVRI=Rkt+ln(V)\ln|V - RI| = -\frac{R}{k}t + \ln(V)
VRI=eRkt+ln(V)V - RI = e^{-\frac{R}{k}t + \ln(V)}
VRI=VeRktV - RI = V e^{-\frac{R}{k}t}
For all t,eRkt>0VRI>0V>RII<VRt, e^{-\frac{R}{k}t} > 0 \Rightarrow V - RI > 0 \Rightarrow V > RI \Rightarrow I < \frac{V}{R}
So, the size of the current can never be greater than VR\frac{V}{R}.

Marking Criteria
DescriptorMarks

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 lnVRI\ln|V - RI| as the subject of the equation

1

expresses relationship as an exponential function

1

considers value of II over time to determine the required limit

1
Q17
2025
QCAA
Paper 1
7 marks
Q17
7 marks

The radius of a cylinder decreases at a constant rate of 0.5 m s10.5 \text{ m s}^{-1}, while maintaining a constant height of four metres.

Given that the cylinder has an initial volume of 100π m3100\pi \text{ m}^3, determine the rate of change of the volume (m3 s1\text{m}^3 \text{ s}^{-1}) of the cylinder after four seconds.

Reveal Answer

Given V=πr2hV=4πr2V = \pi r^2 h \Rightarrow V = 4\pi r^2

Given drdt=0.5 m s1\frac{dr}{dt} = -0.5 \text{ m s}^{-1}

V=4πr2dVdt=8πrdrdtV = 4\pi r^2 \Rightarrow \frac{dV}{dt} = 8\pi r \frac{dr}{dt}

At t=0t = 0, V=100πV = 100\pi (given)

100π=4πr2r=5 m(as r>0)100\pi = 4\pi r^2 \Rightarrow r = 5 \text{ m} (\text{as } r > 0)

At t=4t = 4,

r=54×0.5=3 mr = 5 - 4 \times 0.5 = 3 \text{ m}

When r=3r = 3:

dVdt=8π×3×(0.5)=12π m3 s1\frac{dV}{dt} = 8\pi \times 3 \times (-0.5) = -12\pi \text{ m}^3 \text{ s}^{-1}
Marking Criteria
DescriptorMarks

correctly determines the rule for the volume of cylinder in terms of rr

1

correctly expresses the rate of change of the radius as a mathematical expression

1

determines a general expression for dVdt\frac{dV}{dt}

1

determines initial value of rr

1

determines value of rr after 4 seconds

1

determines value of dVdt\frac{dV}{dt} when t=4t = 4

1

shows logical organisation, having fully attempted the question

1
Q17
2020
QCAA
Paper 2
7 marks
Q17
7 marks

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

vdvdx=9.80.004v2(v0)v\frac{dv}{dx} = 9.8 - 0.004v^2 \quad (v \ge 0)

where vv is the velocity (m s1^{-1}) and xx is the displacement from the ground (m).
Determine the velocity of the object when it strikes the ground.

Reveal Answer

vdvdx=9.80.004v2,v>0v\frac{dv}{dx} = 9.8 - 0.004v^2, v > 0
v9.80.004v2dv=dx\int \frac{v}{9.8 - 0.004v^2} dv = \int dx
10.0080.008v9.80.004v2dv=dx\frac{-1}{0.008} \int \frac{-0.008v}{9.8 - 0.004v^2} dv = \int dx
125ln9.80.004v2=x+c-125 \ln|9.8 - 0.004v^2| = x + c

Given v=0v = 0 when x=100x = -100
125ln9.8=100+c-125 \ln|9.8| = -100 + c
c185.298c \approx -185.298

125ln9.80.004v2=x185.298-125 \ln|9.8 - 0.004v^2| = x - 185.298
Determining vv when x=0x = 0
125ln9.80.004v2=185.298-125 \ln|9.8 - 0.004v^2| = -185.298

Using graph facility of GDC
v36.7 ms1v \approx -36.7 \text{ ms}^{-1} or v36.7 ms1v \approx 36.7 \text{ ms}^{-1}
As v>0v > 0, the negative solution is rejected
v36.7 ms1\therefore v \approx 36.7 \text{ ms}^{-1}

Marking Criteria
DescriptorMarks

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
Q16
2023
QCAA
Paper 2
6 marks
Q16
6 marks

A curve modelled by the relation xy2y+cos1(2x)=1xy^2 - y + \cos^{-1}(2x) = 1, where 0.35x0.27-0.35 \leq x \leq 0.27 and 0y10 \leq y \leq 1, intersects the yy-axis at point AA.

Determine the equation of the tangent to the curve at point AA.

Reveal Answer

Given xy2y+cos1(2x)=1xy^2 - y + \cos^{-1}(2x) = 1

Determining y-coordinate of A
0y+cos1(0)=1y=0.570 - y + \cos^{-1}(0) = 1 \Rightarrow y = 0.57

ddxcos1(2x)=10.25x2\frac{d}{dx}\cos^{-1}(2x) = \frac{-1}{\sqrt{0.25-x^2}}

ddx(xy2)=y2+2xydydx\frac{d}{dx}(xy^2) = y^2 + 2xy\frac{dy}{dx}

Determining dydx\frac{dy}{dx}
ddx(xy2y+cos1(2x))=ddx(1)\frac{d}{dx}\left(xy^2 - y + \cos^{-1}(2x)\right) = \frac{d}{dx}(1)
y2+2xydydxdydx+10.25x2=0y^2 + 2xy\frac{dy}{dx} - \frac{dy}{dx} + \frac{-1}{\sqrt{0.25-x^2}} = 0
dydx(2xy1)=10.25x2y2\frac{dy}{dx}(2xy - 1) = \frac{1}{\sqrt{0.25-x^2}} - y^2
dydx=10.25x2y22xy1\frac{dy}{dx} = \frac{\frac{1}{\sqrt{0.25-x^2}} - y^2}{2xy - 1}

Determining dydx\frac{dy}{dx} at A.
dydx=10.25(0.571)21=1.67\frac{dy}{dx} = \frac{\frac{1}{\sqrt{0.25}} - (0.571)^2}{-1} = -1.67

Determining equation of tangent at A
y=mx+cy = mx + c
y=1.67x+0.57y = -1.67x + 0.57

Marking Criteria
DescriptorMarks

correctly determines y-intercept

1

correctly determines ddxcos1(2x)\frac{d}{dx}\cos^{-1}(2x)

1

correctly determines ddxxy2\frac{d}{dx}xy^2

1

determines an expression for dydx\frac{dy}{dx} using a common factor

1

determines a value for dydx\frac{dy}{dx} at A

1

determines equation of the tangent at A

1
Q18
2023
QCAA
Paper 1
6 marks
Q18
6 marks

A particular solution to the differential equation dydx=x(x2+1)tan(y)\frac{dy}{dx} = \frac{x}{(x^2+1)\tan(y)}, where x0x \ge 0 and π2<y0-\frac{\pi}{2} < y \le 0, passes through the origin.

Determine this solution in the form x=f(y)x = f(y). Leave your answer in simplified form.

Reveal Answer

dydx=x(x2+1)tan(y)\frac{dy}{dx} = \frac{x}{(x^2+1)\tan(y)}

tan(y)dy=xx2+1dx\int \tan(y) dy = \int \frac{x}{x^2+1} dx

sin(y)cos(y)dy=122xx2+1dx-\int \frac{-\sin(y)}{\cos(y)} dy = \frac{1}{2} \int \frac{2x}{x^2+1} dx

lncos(y)=12lnx2+1+c-\ln|\cos(y)| = \frac{1}{2}\ln|x^2+1| + c

Given y=0y=0 when x=0x=0,
lncos(0)=12ln1+c    c=0-\ln|\cos(0)| = \frac{1}{2}\ln|1| + c \implies c=0

12lnx2+1=lncos(y)\therefore \frac{1}{2}\ln|x^2+1| = -\ln|\cos(y)|

lnx2+1=ln1cos(y)\ln\sqrt{x^2+1} = \ln\left|\frac{1}{\cos(y)}\right|

x2+1=sec(y)\sqrt{x^2+1} = |\sec(y)|
x2+1=sec2(y)x^2+1 = \sec^2(y)
x2=tan2(y)x^2 = \tan^2(y)
x=±tan(y)x = \pm \tan(y)

As x0,π2<y0x \ge 0, -\frac{\pi}{2} < y \le 0
x=tan(y)x = -\tan(y)

Marking Criteria
DescriptorMarks

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 xx in terms of yy

1

Evaluates the reasonableness of the results and expresses the solution in the form of x=f(y)x = f(y) in simplified form

1
Q5
2020
QCAA
Paper 2
1 mark
Q5
1 mark

The gradient of the tangent at point A on the curve y2=4xy^2 = 4x is 1.36
The xx-coordinate of A is

A

0.12

B

0.46

C

0.54

D

1.47

Reveal Answer
A

0.12

This value is incorrect and does not satisfy the relationship between the gradient and the coordinates on the curve.

B

0.46

This incorrect value likely results from evaluating m24\frac{m^2}{4} (approx 0.460.46) instead of the correct relationship derived from the derivative.

C

0.54

Correct Answer

Differentiating y2=4xy^2=4x gives dydx=2y\frac{dy}{dx} = \frac{2}{y}. Setting 2y=1.36\frac{2}{y} = 1.36 yields y1.47y \approx 1.47, and substituting this back into x=y24x = \frac{y^2}{4} gives x0.54x \approx 0.54.

D

1.47

This is the yy-coordinate of point A (y=21.361.47y = \frac{2}{1.36} \approx 1.47), but the question asks for the xx-coordinate.

Q7
2024
VCAA
Paper 1
4 marks
Q7
4 marks

Solve the differential equation x+2yx2+1dydx=0x + 2y\sqrt{x^2 + 1}\frac{dy}{dx} = 0, expressing yy as a function of xx, given that y(0)=2y(0) = -2.

Reveal Answer

2ydydx=xx2+1,2ydy=xx2+1dx2y\frac{dy}{dx} = \frac{-x}{\sqrt{x^2 + 1}}, 2y dy = -\frac{x}{\sqrt{x^2 + 1}} dx

2ydy=xx2+1dxy2=x2+1+cy(0)=2c=5y2=5x2+1\begin{align*} \int 2y dy &= \int \frac{-x}{\sqrt{x^2 + 1}} dx\\ y^2 &= -\sqrt{x^2 + 1} + c\\ y(0) &= -2 \Rightarrow c = 5\\ y^2 &= 5 - \sqrt{x^2 + 1} \end{align*}

Thus y=±5x2+1y = \pm\sqrt{5 - \sqrt{x^2 + 1}}. Since y(0)=2y(0) = -2 then y=5x2+1y = -\sqrt{5 - \sqrt{x^2 + 1}}.

Marking Criteria
DescriptorMarks

Provides the correct expression for yy

4

Performs correct integration and finds the constant of integration cc

3

Performs correct integration of both sides

2

Separates variables correctly

1

None of the above

0
Q10
2022
VCAA
Paper 2
1 mark
Q10
1 mark

Consider the curve given by 5x2y3xy+y2=105x^2y - 3xy + y^2 = 10.

The equation of the tangent to this curve at the point (1,m)(1, m), where mm is a real constant, will have a negative gradient when

A

mR[1,0]m \in R \setminus [-1, 0]

B

m=111m = -\sqrt{11} - 1 only

C

mR(1,0]m \in R \setminus (-1, 0]

D

m=111m = \sqrt{11} - 1 only

E

m=111m = -\sqrt{11} - 1 or m=111m = \sqrt{11} - 1

Reveal Answer
A

mR[1,0]m \in R \setminus [-1, 0]

While this is the range of mm values that yield a negative gradient, it fails to account for the fact that the point (1,m)(1, m) must actually lie on the given curve.

B

m=111m = -\sqrt{11} - 1 only

This is only one of the two valid values for mm that lie on the curve and produce a negative gradient.

C

mR(1,0]m \in R \setminus (-1, 0]

This inequality is incorrect for the gradient condition, and it also ignores that the point (1,m)(1, m) must satisfy the curve's equation.

D

m=111m = \sqrt{11} - 1 only

This is only one of the two valid values for mm that lie on the curve and produce a negative gradient.

E

m=111m = -\sqrt{11} - 1 or m=111m = \sqrt{11} - 1

Correct Answer

Substituting (1,m)(1, m) into the curve's equation gives m2+2m10=0m^2 + 2m - 10 = 0, yielding m=1±11m = -1 \pm \sqrt{11}. Both of these values satisfy the condition for a negative gradient, which is dydx=7m2m+2<0\frac{dy}{dx} = \frac{-7m}{2m+2} < 0.

Q16
2025
QCAA
Paper 2
6 marks
Q16
6 marks

A certain population can be approximately modelled by the differential equation

dPdt=0.5P(10.2P)\frac{dP}{dt} = 0.5P(1 - 0.2P)

where PP is the population in millions and tt 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 dPdt=0.5P(10.2P)\frac{dP}{dt} = 0.5P(1 - 0.2P):

1P(10.2P) dP=0.5 dt(1)\int \frac{1}{P(1 - 0.2P)} \ dP = \int 0.5 \ dt \quad \dots (1) 1P(10.2P)=AP+B(10.2P)(2)A(10.2P)+BP=1\begin{align*} \frac{1}{P(1 - 0.2P)} &= \frac{A}{P} + \frac{B}{(1 - 0.2P)} \quad \dots (2)\\ A(1 - 0.2P) + BP &= 1 \end{align*}

P=0:A=1P = 0 : A = 1
P=5:B=0.2P = 5 : B = 0.2

From (1) and (2):

1P+0.2(10.2P) dP=0.5 dtlnPln10.2P=0.5t+c\begin{align*} \int \frac{1}{P} + \frac{0.2}{(1 - 0.2P)} \ dP = \int 0.5 \ dt\\ \ln|P| - \ln|1 - 0.2P| = 0.5t + c \end{align*}

Given P=0.3P = 0.3 when t=0t = 0

ln(0.3)ln(10.2×0.3)=cc1.142\ln(0.3) - \ln(1 - 0.2 \times 0.3) = c \Rightarrow c \approx -1.142

ln(P)ln(10.2P)=0.5t1.142\ln(P) - \ln(1 - 0.2P) = 0.5t - 1.142
When t=5t = 5
ln(P)ln(10.2P)=0.5×51.142=1.358\ln(P) - \ln(1 - 0.2P) = 0.5 \times 5 - 1.142 = 1.358

Using GDC: P2.19P \approx 2.19
The estimated population on 1 January 2030 is 2.2 million.

Marking Criteria
DescriptorMarks

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 PP whose solution represents the required population

1

estimates the required population

1
Q8
2022
QCAA
Paper 2
1 mark
Q8
1 mark

Determine the gradient of the tangent to the curve y23x=5y^2 - 3x = 5 at the point (1,22)(1, 2\sqrt{2}).

A

0.41

B

0.53

C

1.06

D

8.49

Reveal Answer
A

0.41

This value is incorrect. It does not match the result derived from implicit differentiation.

B

0.53

Correct Answer

Differentiating y23x=5y^2 - 3x = 5 implicitly gives 2ydydx3=02y \frac{dy}{dx} - 3 = 0. Solving for the gradient yields dydx=32y\frac{dy}{dx} = \frac{3}{2y}. Substituting y=22y = 2\sqrt{2} gives 3420.53\frac{3}{4\sqrt{2}} \approx 0.53.

C

1.06

This answer results from incorrectly differentiating y2y^2 as ydydxy \frac{dy}{dx} instead of 2ydydx2y \frac{dy}{dx}, leading to 3y1.06\frac{3}{y} \approx 1.06.

D

8.49

This value is incorrect. It appears to result from multiplying 33 by yy (3×228.493 \times 2\sqrt{2} \approx 8.49) rather than dividing 33 by 2y2y.

Q10
2020
VCAA
Paper 2
1 mark
Q10
1 mark

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, mm grams, of salt in the tank at time tt minutes, for a non-zero volume of mixture, is

A

dmdt=0\frac{dm}{dt}=0

B

dmdt=5m505t\frac{dm}{dt}=-\frac{5m}{50-5t}

C

dmdt=30m10\frac{dm}{dt}=30-\frac{m}{10}

D

dmdt=305m503t\frac{dm}{dt}=30-\frac{5m}{50-3t}

E

dmdt=305m505t\frac{dm}{dt}=30-\frac{5m}{50-5t}

Reveal Answer
A

dmdt=0\frac{dm}{dt}=0

This implies the mass of salt is constant, completely ignoring both the inflow and outflow of salt from the tank.

B

dmdt=5m505t\frac{dm}{dt}=-\frac{5m}{50-5t}

This ignores the inflow of salt (3030 g/min) and incorrectly calculates the volume as 505t50-5t instead of 503t50-3t.

C

dmdt=30m10\frac{dm}{dt}=30-\frac{m}{10}

This assumes the volume of the tank is constant at 5050 L, leading to an outflow rate of 5m50=m10\frac{5m}{50} = \frac{m}{10}, but the volume is actually decreasing by 33 L/min.

D

dmdt=305m503t\frac{dm}{dt}=30-\frac{5m}{50-3t}

Correct Answer

The rate of salt entering is 15×2=3015 \times 2 = 30 g/min, and the rate leaving is the outflow rate (55 L/min) times the concentration m503t\frac{m}{50-3t}.

E

dmdt=305m505t\frac{dm}{dt}=30-\frac{5m}{50-5t}

This incorrectly calculates the volume of the mixture at time tt as 505t50-5t, which only accounts for the outflow rate and ignores the 22 L/min inflow.

Q4
2023
VCAA
Paper 1
3 marks
Q4
3 marks

Consider the relation xarcsin(y2)=πx \arcsin(y^2) = \pi.

Use implicit differentiation to find dydx\frac{dy}{dx} at the point (6,12)\left(6, \frac{1}{\sqrt{2}}\right).

Give your answer in the form πab-\frac{\pi\sqrt{a}}{b}, where a,bZ+a, b \in Z^+.

Reveal Answer

Using implicit differentiation,

arcsin(y2)+x×2y1y4dydx=0\arcsin(y^2) + x \times \frac{2y}{\sqrt{1-y^4}} \frac{dy}{dx} = 0

At the point (6,12)\left(6, \frac{1}{\sqrt{2}}\right),

arcsin(12)+6×2114dydx=0π6+1223dydx=0\begin{align*} \arcsin\left(\frac{1}{2}\right) + 6 \times \frac{\sqrt{2}}{\sqrt{1-\frac{1}{4}}} \frac{dy}{dx} &= 0\\ \Rightarrow \frac{\pi}{6} + \frac{12\sqrt{2}}{\sqrt{3}} \frac{dy}{dx} &= 0 \end{align*}

and so

dydx=π6×3122=π6×624=π6144\frac{dy}{dx} = -\frac{\pi}{6} \times \frac{\sqrt{3}}{12\sqrt{2}} = -\frac{\pi}{6} \times \frac{\sqrt{6}}{24} = -\frac{\pi\sqrt{6}}{144}
Marking Criteria
DescriptorMarks

Correctly differentiates the relation implicitly, demonstrating appropriate use of the product and chain rules (e.g. arcsin(y2)+x2y1y4dydx=0\arcsin(y^2) + x \frac{2y}{\sqrt{1-y^4}} \frac{dy}{dx} = 0)

1

Correctly substitutes x=6x = 6 and y=12y = \frac{1}{\sqrt{2}} into the differentiated equation

1

Correctly evaluates to find the final answer in the required form, dydx=π6144\frac{dy}{dx} = -\frac{\pi\sqrt{6}}{144}

1
Q13
2024
SCSA
Paper 2
8 marks
Q13

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, P(t)P(t), of brumbies in Kosciuszko National Park tt years since the ban, by

P(t)=1800010.25e0.15t+1\begin{align*} P(t) = \frac{18000}{10.25e^{-0.15t} + 1} \end{align*}
Q13c

It can be shown that the growth rate of the population of brumbies can be expressed as

dPdt=1rP(kP)\frac{dP}{dt} = \frac{1}{r}P(k - P).

Q13a
1 mark

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 P(t)=4800P(t) = 4800\quad i.e. 4800=1800010.25e0.15t+14800 = \frac{18000}{10.25e^{-0.15t} + 1}

From CAS t=8.7711yearst = 8.7711\dots \text{years}

Marking Criteria
DescriptorMarks

solves for tt correctly

1
Q13b
2 marks

From this model, determine the estimated long run number of brumbies in Kosciuszko National Park.

Reveal Answer

Using tt \to \infty then e0.15t0,e^{-0.15t} \to 0, hence, P(t)1800010.25(0)+1=18000P(t) \to \frac{18000}{10.25(0) + 1} = 18000.

Hence in the long term, the limiting population will be 18 000 brumbies.

Marking Criteria
DescriptorMarks

considers tt \to \infty to use e0.15t0e^{-0.15t} \to 0

1

determines the long run number of brumbies

1
Q13c
3 marks

Determine the values of the constants rr and kk.

Reveal Answer

k=18000k = 18000 as this is the limiting population.

dPdt=1rP(18000P)\therefore \frac{dP}{dt} = \frac{1}{r}P(18000 - P)

Using P(t)=1800010.25e0.15t+1P(t) = \frac{18000}{10.25e^{-0.15t} + 1} from CAS P(0)=218.666P'(0) = 218.666\dots (when P=1600P = 1600)

Hence substituting into dPdt=1rP(18000P)\frac{dP}{dt} = \frac{1}{r}P(18000 - P)

i.e. 218.666=1r(1600)(180001600)218.666\dots = \frac{1}{r}(1600)(18000 - 1600)

r=120000\therefore r = 120000

Marking Criteria
DescriptorMarks

states the value of kk correctly

1

states the value of rr correctly

1

provides justification for the determination of the value for rr

1
Q13d
2 marks

Determine the greatest growth rate for the population of brumbies.

Reveal Answer

Greatest rate of growth will occur when P=9000P = 9000 (half the limiting population)

Using P=9000,dPdt=1120000(9000)(180009000)=675 brumbies per yearP = 9000, \quad \frac{dP}{dt} = \frac{1}{120000}(9000)(18000 - 9000) = 675 \text{ brumbies per year}

Hence greatest growth rate will be 675 brumbies per year.

Marking Criteria
DescriptorMarks

states that the maximum growth rate occurs when P=0.5k=9000P = 0.5k = 9000

1

calculates the maximum growth rate correctly and states the correct units

1
Q1
2025
VCAA
Paper 1
4 marks
Q1
4 marks

Consider the curve with equation xe2y+y2ex=8e4xe^{-2y} + y^2e^x = 8e^4.

Find the equation of the tangent to the curve at the point (4,2)(4, -2).

Reveal Answer

Method 1
x(2e2y)dydx+e2y+2yexdydx+y2ex=0x\left(-2e^{-2y}\right)\frac{dy}{dx} + e^{-2y} + 2ye^x\frac{dy}{dx} + y^2e^x = 0

dydx=e2yy2ex2xe2y+2yex\frac{dy}{dx} = \frac{-e^{-2y} - y^2e^x}{-2xe^{-2y} + 2ye^x}

At (4,2)(4, -2):

dydx=e44e48e44e4\frac{dy}{dx} = \frac{-e^4 - 4e^4}{-8e^4 - 4e^4}

dydx=52\frac{dy}{dx} = \frac{5}{2}

y=512x113y = \frac{5}{12}x - \frac{11}{3}

Marking Criteria
DescriptorMarks

Differentiates implicitly to obtain x(2e2y)dydx+e2y+2yexdydx+y2ex=0x\left(-2e^{-2y}\right)\frac{dy}{dx} + e^{-2y} + 2ye^x\frac{dy}{dx} + y^2e^x = 0 or equivalent

1

Rearranges the equation to make dydx\frac{dy}{dx} the subject

1

Substitutes x=4x=4 and y=2y=-2 to calculate the gradient dydx=512\frac{dy}{dx} = \frac{5}{12}

1

Determines the correct equation of the tangent y=512x113y = \frac{5}{12}x - \frac{11}{3}

1

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