SCSA Mathematics Methods The logarithmic function

15 sample questions with marking guides and sample answers · Avg. score: 63.1%

Q12
2021
QCAA
Paper 1
5 marks
Q12

Solve for xx in the following.

Q12a
2 marks

log2(5x+7)=5\log_2(5x+7)=5

Reveal Answer

Changing from log to index form
5x+7=325x + 7 = 32
5x=255x = 25
x=5x = 5

Marking Criteria
DescriptorMarks

correctly establishes the linear equation

1

determines x

1
Q12b
3 marks

log10(x+3)+log10(x3)=log10(9x29)\log_{10}(x+3)+\log_{10}(x-3)=\log_{10}(9x-29)

Reveal Answer

Using addition log law
log10(x29)=log10(9x29)\log_{10}(x^2 - 9) = \log_{10}(9x - 29)
Equating and rearranging
x29x+20=0x^2 - 9x + 20 = 0
Factorising
(x4)(x5)=0(x - 4)(x - 5) = 0

x=4,5\rightarrow x = 4, 5

Marking Criteria
DescriptorMarks

correctly applies the log law

1

establishes quadratic equation

1

determines 2 solutions

1
Q6
2020
SCSA
Paper 1
7 marks
Q6

Consider the function f(x)=ln(x)f(x) = \ln(x). The function g(x)=f(x)+ag(x) = f(x) + a is a vertical translation of ff by aa units.

Q6a
2 marks

Express the function g(x)=ln(4x)g(x) = \ln(4x) in terms of a vertical translation of ff (i.e. in the form g(x)=f(x)+ag(x) = f(x) + a), stating the number of units that ff is translated.

Reveal Answer

g(x)=ln(4x)=ln(4)+ln(x)=f(x)+ln(4)\begin{align*} g(x) &= \ln(4x)\\ &= \ln(4) + \ln(x)\\ &= f(x) + \ln(4) \end{align*}

ff is translated vertically (upward) by ln(4)\ln(4) units.

Marking Criteria
DescriptorMarks

expresses g(x)g(x) as a sum of logs

1

recognises a vertical translation by ln(4)\ln(4) units

1
Q6b
2 marks

The function h(x)=cf(x)h(x) = cf(x) is a vertical dilation of ff by a scale factor of cc.

Express the function h(x)=ln(x)h(x) = \ln(\sqrt{x}) in terms of a vertical dilation of ff, stating the scale factor.

Reveal Answer

h(x)=ln(x)=ln(x0.5)=0.5ln(x)=0.5f(x)\begin{align*} h(x) &= \ln(\sqrt{x})\\ &= \ln(x^{0.5})\\ &= 0.5 \ln(x)\\ &= 0.5 f(x) \end{align*}

ff is scaled vertically by a factor of 0.5.

Marking Criteria
DescriptorMarks

expresses hh as a product involving ln(x)\ln(x)

1

recognises a vertical scaling by a scale factor of 0.5

1
Q6c
3 marks

The function p(x)=f(bx)p(x) = f(bx) is a horizontal dilation of ff by a scale factor of 1b\frac{1}{b}.

Express the function p(x)=ln(x)+4p(x) = \ln(x) + 4 in terms of a horizontal dilation of ff, stating the scale factor.

Reveal Answer

p(x)=ln(x)+4=ln(x)+4ln(e)=ln(x)+ln(e4)=ln(e4x)=f(e4x)\begin{align*} p(x) &= \ln(x) + 4\\ &= \ln(x) + 4\ln(e)\\ &= \ln(x) + \ln(e^4)\\ &= \ln(e^4x)\\ &= f(e^4x) \end{align*}

ff is scaled horizontally by a scale factor of e4e^{-4}.

Marking Criteria
DescriptorMarks

expresses 4 as 4ln(e)4\ln(e)

1

expresses pp using a single logarithm

1

states horizontal scale factor

1
Q2
2023
VCAA
Paper 1
3 marks
Q2
3 marks

Solve e2x12=4exe^{2x} - 12 = 4e^x for xRx \in R.

Reveal Answer

e2x4ex12=0e^{2x} - 4e^x - 12 = 0

m24m12=0m^2 - 4m - 12 = 0 where m=exm = e^x

(m6)(m+2)=0(m - 6)(m + 2) = 0

m=6m = 6 or m=2m = -2

ex=6e^x = 6 or ex=2e^x = -2

x=loge6\therefore x = \log_e 6

Marking Criteria
DescriptorMarks

Forms a quadratic equation in terms of exe^x (e.g., e2x4ex12=0e^{2x} - 4e^x - 12 = 0)

1

Solves the quadratic equation to find ex=6e^x = 6 and ex=2e^x = -2

1

States the correct final answer x=loge6x = \log_e 6, correctly discarding the invalid solution

1
Q2
2020
QCAA
Paper 2
1 mark
Q2
1 mark

The pH of a substance is a measure of its acidity and is given by the formula pH=log10[H+]\text{pH} = -\log_{10}[\text{H}^+] where [H+][\text{H}^+] is the concentration of hydrogen ions in moles per litre. If a solution has a pH equal to 0.2, the concentration of hydrogen ions in moles per litre is closest to

A

0.32

B

0.63

C

0.70

D

1.58

Reveal Answer
A

0.32

This value is incorrect. It is close to 100.510^{-0.5} rather than the required 100.210^{-0.2}.

B

0.63

Correct Answer

Rearranging the formula pH=log10[H+]\text{pH} = -\log_{10}[\text{H}^+] gives [H+]=10pH[\text{H}^+] = 10^{-\text{pH}}. Substituting the given pH, [H+]=100.20.63[\text{H}^+] = 10^{-0.2} \approx 0.63.

C

0.70

This value is incorrect and does not result from solving the logarithmic equation for [H+][\text{H}^+].

D

1.58

This result comes from calculating 100.210^{0.2}, which incorrectly ignores the negative sign in the pH formula.

Q9
2021
QCAA
Paper 2
1 mark
Q9
1 mark

The graphs of the functions f(x)=2ex+5f(x) = 2e^x + 5 and g(x)=3exg(x) = \frac{3}{e^x} intersect at point A. Determine the coordinates of point A.

A

(1.609, 15)

B

(1.099, 1)

C

(0.4065, 2)

D

(-0.693, 6)

Reveal Answer
A

(1.609, 15)

Incorrect. While this point lies on the graph of f(x)f(x) (since xln(5)x \approx \ln(5) gives f(x)=15f(x) = 15), g(1.609)0.6g(1.609) \approx 0.6. The intersection point must satisfy both equations.

B

(1.099, 1)

Incorrect. This point lies on the graph of g(x)g(x) (since xln(3)x \approx \ln(3) gives g(x)=1g(x) = 1), but f(1.099)11f(1.099) \approx 11. The functions are not equal at this x-value.

C

(0.4065, 2)

Incorrect. This point lies on the graph of g(x)g(x) (since xln(1.5)x \approx \ln(1.5) gives g(x)=2g(x) = 2), but f(0.4065)8f(0.4065) \approx 8. The y-values differ significantly.

D

(-0.693, 6)

Correct Answer

Correct. Set 2ex+5=3ex2e^x + 5 = \frac{3}{e^x} and multiply by exe^x to get the quadratic 2(ex)2+5ex3=02(e^x)^2 + 5e^x - 3 = 0. Factoring gives (2ex1)(ex+3)=0(2e^x - 1)(e^x + 3) = 0. Since ex>0e^x > 0, ex=12e^x = \frac{1}{2}, which yields x=ln(0.5)0.693x = \ln(0.5) \approx -0.693 and y=6y = 6.

Q5
2022
VCAA
Paper 1
5 marks
Q5a
2 marks

Solve 103x13=10010^{3x-13} = 100 for xx.

Reveal Answer

10(3x13)=10210^{(3x-13)} = 10^2
3x13=23x - 13 = 2
x=5x = 5

Marking Criteria
DescriptorMarks

Expresses 100100 as 10210^2 and equates the exponents, e.g., 3x13=23x - 13 = 2

1

Solves for xx to find the correct answer, x=5x = 5

1
Q5b
3 marks

Find the maximal domain of ff, where f(x)=loge(x22x3)f(x) = \log_e(x^2 - 2x - 3).

Reveal Answer

Maximal domain when x22x3>0x^2 - 2x - 3 > 0
(x3)(x+1)>0(x - 3)(x + 1) > 0
From NFL x<1x < -1 and x>3x > 3
x(,1)(3,)x \in (-\infty, -1) \cup (3, \infty)

Marking Criteria
DescriptorMarks

Identifies that the argument of the logarithm must be strictly positive, x22x3>0x^2 - 2x - 3 > 0

1

Factorises the quadratic expression or finds the critical values, e.g., (x3)(x+1)>0(x - 3)(x + 1) > 0

1

States the correct maximal domain using union notation, x(,1)(3,)x \in (-\infty, -1) \cup (3, \infty)

1
Q15
2020
QCAA
Paper 1
4 marks
Q15

Solve the following equations.

Q15a
1 mark

4ex=1004e^x = 100

Reveal Answer

ex=25e^x = 25
x=ln(25)x = ln(25)

Marking Criteria
DescriptorMarks

correctly determines x=ln(25)x = ln(25)

1
Q15b
3 marks

2log4xlog4(x1)=12 \log_4 x - \log_4 (x - 1) = 1

Reveal Answer

Using log laws
log4(x2x1)=1log_4(\frac{x^2}{x-1}) = 1
Change from log to index form
x2x1=4\frac{x^2}{x-1} = 4
x24x+4=0x^2 - 4x + 4 = 0
x=2x = 2

Marking Criteria
DescriptorMarks

correctly establishes equation using log laws

1

correctly establishes the quadratic equation

1

determines x

1
Q14
2024
QCAA
Paper 2
6 marks
Q14

A football coach offered a 12-day intensive training clinic. During the clinic, the height that each player could kick a football was monitored.
One player's kick heights could be modelled by H(t)=log10(10t+10)+5H(t) = \log_{10}(10t + 10) + 5, 0t120 \le t \le 12, where H(t)H(t) is vertical height (m) and tt is the time (days) spent in training.

Q14a
1 mark

Determine the initial height that the player could kick the ball.

Reveal Answer

H(0)=6 mH(0)=6\ \text{m}

Marking Criteria
DescriptorMarks

Correctly determines the initial height

1
Q14b
1 mark

Determine the training time needed for the player to be able to kick the ball to a height of 7 m.

Reveal Answer

Using a GDC:

t=9 dayst=9\ \text{days}

Marking Criteria
DescriptorMarks

Correctly determines the time required

1
Q14c
2 marks

Determine the overall improvement in kick height achieved by completing the clinic.

Reveal Answer

Initially, the kick height was 6 metres.

At the end of the course, t=12t=12, the kick height increased
to 7.1139 metres.

The kick height has increased by 1.1139 metres during the
course.

Marking Criteria
DescriptorMarks

Correctly determines the kick height at the end of the course

1

Determines the overall kick height improvement

1
Q14d
1 mark

Determine the rate of change in kick height when t=1.5t = 1.5 days.

Reveal Answer

Using a GDC:

H(1.5)=0.17372 m/dayH'(1.5)=0.17372\ \text{m/day}

Marking Criteria
DescriptorMarks

Correctly determines the derivative value when t=1.5t=1.5

1
Q14e
1 mark

Determine the training time (as a decimal) when the rate of change in kick height is 0.09 m/day.

Reveal Answer

Using a GDC:

Graph derivative function and y=0.09y=0.09

Find point of intersection.

H(t)=0.09H'(t)=0.09

t=3.82549 dayst=3.82549\ \text{days}

Marking Criteria
DescriptorMarks

Correctly determines the time as a decimal

1
Q1
2023
QCAA
Paper 1
1 mark
Q1
1 mark

eln(x)e^{\ln(x)} is equivalent to

A

0

B

1

C

xx

D

1x\frac{1}{x}

Reveal Answer
A

0

This is incorrect. The expression evaluates to xx, not the constant 0. Furthermore, ln(x)\ln(x) is undefined at x=0x=0.

B

1

This is incorrect. The expression would only equal 1 if x=1x=1 (since ln(1)=0\ln(1)=0 and e0=1e^0=1), but it is not equal to 1 for all values of xx.

C

xx

Correct Answer

This is correct. The natural logarithm function ln(x)\ln(x) and the exponential function exe^x are inverse functions, so eln(x)=xe^{\ln(x)} = x for all x>0x > 0.

D

1x\frac{1}{x}

This is incorrect. This result corresponds to eln(x)=eln(1/x)=1xe^{-\ln(x)} = e^{\ln(1/x)} = \frac{1}{x}, not the original expression.

Q19
2020
QCAA
Paper 1
6 marks
Q19
6 marks

A horizontal point of inflection is a point of inflection that is also a stationary point.

Determine the value/s of kk for which the graph of f(x)=ln(x)kkxx+1f(x) = \frac{\ln(x)}{k} - \frac{kx}{x+1} has only one horizontal point of inflection.

Reveal Answer

f(x)=1kx(k(x+1)kx(x+1)2)f'(x) = \frac{1}{kx} - (\frac{k(x + 1) - kx}{(x + 1)^2})
Stationary points f(x)=0f'(x) = 0

0=1kx(k(x+1)kx(x+1)2)0 = \frac{1}{kx} - (\frac{k(x + 1) - kx}{(x + 1)^2})

0=1kxk(x+1)20 = \frac{1}{kx} - \frac{k}{(x + 1)^2}

0=x2+(2k2)x+10 = x^2 + (2 - k^2)x + 1 (i)

The quadratic has real roots when discriminant 0\ge 0

(2k2)240(2 - k^2)^2 - 4 \ge 0
2k2±22 - k^2 \ge \pm 2
There is only ONE phi \therefore
2k2=±22 - k^2 = \pm 2
k=0k = 0 (not valid)
and k2=4k^2 = 4 so
k=2,2k = 2, -2
Sub into (i) to determine the x-ordinate of the stationary point.
x=1\rightarrow x = 1
For k=2k = 2
f(x)=12x2+4(x+1)3\therefore f''(x) = \frac{-1}{2x^2} + \frac{4}{(x + 1)^3}
f(1)=12+48f''(1) = \frac{-1}{2} + \frac{4}{8}
f(1)=0f''(1) = 0
For k=2k = -2
f(x)=12x24(x+1)3f''(x) = \frac{1}{2x^2} - \frac{4}{(x + 1)^3}
f(1)=1248f''(1) = \frac{1}{2} - \frac{4}{8}
f(1)=0f''(1) = 0
For each kk value, x=1x = 1 is the x-ordinate of both a stationary point (f(x)=0f'(x) = 0) and a point of inflection (f(x)=0f''(x) = 0)
There is a point of horizontal inflection at x=1x = 1 when k=±2k = \pm 2

Marking Criteria
DescriptorMarks

correctly determines the first derivative

1

correctly determines the quadratic equation to identify the stationary point/s

1

determines valid and non-valid solutions of k

1

determines x-ordinate of stationary point

1

determines values of second derivative for both values of k

1

shows logical organisation communicating key steps

1
Q5
2023
QCAA
Paper 1
1 mark
Q5
1 mark

Determine 1312xdx\int_1^3 \frac{1}{2x} dx.

A

12ln6\frac{1}{2} \ln 6

B

12ln5\frac{1}{2} \ln 5

C

12ln4\frac{1}{2} \ln 4

D

12ln3\frac{1}{2} \ln 3

Reveal Answer
A

12ln6\frac{1}{2} \ln 6

This answer is incorrect. It corresponds to evaluating an antiderivative like 12ln(2x)\frac{1}{2}\ln(2x) only at the upper limit x=3x=3, neglecting to subtract the value at the lower limit.

B

12ln5\frac{1}{2} \ln 5

This is incorrect. The evaluation of the definite integral depends on the limits 11 and 33, which results in ln(3)\ln(3), not ln(5)\ln(5).

C

12ln4\frac{1}{2} \ln 4

This option is incorrect. It may result from an arithmetic error or misapplying logarithmic properties, as the integral yields ln(3)ln(1)\ln(3) - \ln(1), not ln(4)\ln(4).

D

12ln3\frac{1}{2} \ln 3

Correct Answer

We factor out the constant to get 12131xdx\frac{1}{2}\int_1^3 \frac{1}{x} dx. The antiderivative is 12[lnx]13=12(ln3ln1)\frac{1}{2}[\ln|x|]_1^3 = \frac{1}{2}(\ln 3 - \ln 1), which simplifies to 12ln3\frac{1}{2}\ln 3 since ln1=0\ln 1 = 0.

Q8
2023
QCAA
Paper 2
1 mark
Q8
1 mark

The number of koalas in a conservation park is modelled by N=15ln(7t+1)N = 15 \ln(7t + 1), t1t \geq 1, where tt represents the time (years) since the park opened. There were 20 koalas in the park when it opened.

Determine the approximate rate of change in the number of koalas when t=3t = 3.

A

46

B

26

C

25

D

5

Reveal Answer
A

46

This is the value of the function N(3)=15ln(22)46N(3) = 15 \ln(22) \approx 46. This represents the number of koalas (or the population increase) at year 3, rather than the rate at which the population is changing.

B

26

This value appears to be the result of calculating N(3)2026N(3) - 20 \approx 26. This subtracts the initial population from the model's value at t=3t=3, which does not represent the instantaneous rate of change.

C

25

This is an incorrect value. It does not correspond to the derivative at t=3t=3 or the function value, likely resulting from a calculation error.

D

5

Correct Answer

The rate of change is found by taking the derivative dNdt\frac{dN}{dt}. Using the chain rule, dNdt=1517t+17=1057t+1\frac{dN}{dt} = 15 \cdot \frac{1}{7t+1} \cdot 7 = \frac{105}{7t+1}. Evaluating at t=3t=3 gives 105224.77\frac{105}{22} \approx 4.77, which rounds to 5.

Q7
2022
QCAA
Paper 2
1 mark
Q7
1 mark

A marble moves in one direction in a straight line with velocity v=2ln(t+1)v = 2\ln(t + 1) (in metres per second) where tt is time (in seconds) since the marble passed through the origin.

Determine the distance from the origin the marble has rolled after 4 seconds.

A

0.40 m

B

1.60 m

C

3.22 m

D

8.09 m

Reveal Answer
A

0.40 m

This value represents the acceleration at t=4t=4, found by differentiating the velocity function: a(4)=v(4)=24+1=0.40 m/s2a(4) = v'(4) = \frac{2}{4+1} = 0.40 \text{ m/s}^2.

B

1.60 m

This value corresponds to ln(5)\ln(5), which is an intermediate value in the calculation but not the final integral result.

C

3.22 m

This is the instantaneous velocity of the marble at t=4t=4, calculated as v(4)=2ln(5)3.22 m/sv(4) = 2\ln(5) \approx 3.22 \text{ m/s}, rather than the total distance traveled.

D

8.09 m

Correct Answer

Distance is found by integrating velocity from t=0t=0 to t=4t=4: 042ln(t+1)dt=[2((t+1)ln(t+1)(t+1))]04=10ln(5)88.09 m\int_0^4 2\ln(t+1) \, dt = [2((t+1)\ln(t+1) - (t+1))]_0^4 = 10\ln(5) - 8 \approx 8.09 \text{ m}.

Q10
2020
VCAA
Paper 2
1 mark
Q10
1 mark

Given that log2(n+1)=x\log_2(n + 1) = x, the values of nn for which xx is a positive integer are

A

n=2kn = 2^k, kZ+k \in Z^+

B

n=2k1n = 2^k - 1, kZ+k \in Z^+

C

n=2k1n = 2^{k - 1}, kZ+k \in Z^+

D

n=2k1n = 2k - 1, kZ+k \in Z^+

E

n=2kn = 2k, kZ+k \in Z^+

Reveal Answer
A

n=2kn = 2^k, kZ+k \in Z^+

If n=2kn = 2^k, then x=log2(2k+1)x = \log_2(2^k + 1), which does not result in an integer value for xx.

B

n=2k1n = 2^k - 1, kZ+k \in Z^+

Correct Answer

Converting the logarithmic equation to exponential form gives n+1=2xn + 1 = 2^x. Substituting xx with a positive integer kk yields n=2k1n = 2^k - 1.

C

n=2k1n = 2^{k - 1}, kZ+k \in Z^+

If n=2k1n = 2^{k-1}, then x=log2(2k1+1)x = \log_2(2^{k-1} + 1), which does not evaluate to an integer for positive integers kk.

D

n=2k1n = 2k - 1, kZ+k \in Z^+

If n=2k1n = 2k - 1, then x=log2(2k)x = \log_2(2k), which is only an integer when kk itself is a power of 2, not for all positive integers kk.

E

n=2kn = 2k, kZ+k \in Z^+

If n=2kn = 2k, then x=log2(2k+1)x = \log_2(2k + 1), which does not yield an integer for positive integers kk.

Q10
2024
QCAA
Paper 1
1 mark
Q10
1 mark

Given that log106=0.778\log_{10} 6 = 0.778, determine the value of log10600\log_{10} 600

A

77.800

B

10.778

C

2.778

D

1.556

Reveal Answer
A

77.800

This result comes from incorrectly multiplying the given logarithm by 100 (0.778×1000.778 \times 100). However, log10(100×6)=log106+log10100\log_{10}(100 \times 6) = \log_{10} 6 + \log_{10} 100, which involves addition, not multiplication.

B

10.778

This option incorrectly adds 10 to the given value. Since 600=6×102600 = 6 \times 10^2, you must add the exponent 2 (because log10100=2\log_{10} 100 = 2), not the base 10.

C

2.778

Correct Answer

Using the product rule for logarithms, log10600=log10(6×100)=log106+log10100\log_{10} 600 = \log_{10}(6 \times 100) = \log_{10} 6 + \log_{10} 100. Substituting the values gives 0.778+2=2.7780.778 + 2 = 2.778.

D

1.556

This value represents 2×log1062 \times \log_{10} 6, which equals log10(62)=log1036\log_{10}(6^2) = \log_{10} 36. The logarithm of a product (6×1006 \times 100) requires adding the logs, not multiplying the log value by 2.

Frequently Asked Questions

How many SCSA Mathematics Methods questions cover The logarithmic function?
AusGrader has 167 SCSA Mathematics Methods questions on The logarithmic function, all with instant AI grading and detailed marking feedback.

Ready to practise SCSA Mathematics Methods?

Get instant AI feedback on past exam questions, aligned to the syllabus

Start Practising Free