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VCE 12 Methods 2023

4.06 Equations with logarithms

Worksheet
Logarithmic equations
1

Solve for y in each of the following logarithmic equations:

a

\log_{7} y = 2

b

\log_{16} y = \dfrac{1}{2}

c

\log_{y} 8 = 3

d

2\log_{2} y + 3\log_{2} y = 15

e

\log_{6} y^2 = 4

f

\log_{2} y^{4} + \log_{2} y = 10

2

Solve for x in each of the following logarithmic equations:

a

\log_{2} 16 = x

b

\log_{12} 144 = x

c

\log_{3} x = 4

d

2 \log x = 4

e

\log_{0.1} x = 4

f

\log_{16} x = \dfrac{1}{4}

g

\log_{x} 64 = 2

h

\log_{x} 125 = 3

i

\log_{2} \left( 7 x + 5\right) = 4

j

\log_{6} \left( 3 x - 9\right) = 2

k

\log_{10} \left( 3 x + 982\right) = 3

l

6 \log_{4} \left( 2 x\right) - 18 = 0

m

11 \log_{5} \left(x - 12\right) = 33

n

\log_{2} \left(\sqrt{ 2 x^{3}}\right) + 1 = 4.5

o

\log_{2} \left(\log_{2} x\right) = 0

p

\log_{2} \left( 3 x + 35\right) = \log_{2} 5

q

\log_{9} \left( 10 x - 3\right) = \log_{9} \left( 5 x + 4\right)

3

Solve for x in each of the following logarithmic equations:

a

\log_{7} \left( 9 x - 2\right) = \log_{7} \left( 8 x + 9\right)

b

\log_{6} x = \sqrt{\log_{6} x}

c

\log_{2} x - 4 = \sqrt{\log_{2} x - 4}

d

3 \log x = \log 125

e

\log_{10} x + \log_{10} 6 = \log_{10} 48

f

\log_{7} 2 x + \log_{7} 3 = 3

g

\log_{4} 45 - \log_{4} 9 x = 2

h

\log_{4} x = \log_{4} 7 - \log_{4} \left(x - 4\right)

i

\log x + \log \left(x + 3\right) = 1

j

\log_{3} x + \log_{3} 25 x = 8

k

\log_{5} \left( 4 x^{2}\right) - \log_{5} x = 3

l

\log \left(x + 12\right) - \log \left(x + 5\right) = \log x

m

\log_{8} \left( 5 x + 12\right) = \log_{8} \left(x + 6\right) - \log_{8} 3

n

\log \left(x + 5\right) + \log \left(x - 2\right) = \log 8

o

\log_{4} \left(x + 3\right) + \log_{4} \left(x - 3\right) = 2

Exponential equations and logarithms
4

Find the interval in which the solution of the following equations will lie:

a

5^{x} = 12

b

3^{x} = 29

c

2^{x} = \dfrac{1}{13}

d
2^{x} = - 5
5

For each of the following equations:

i

Make x the subject of the equation.

ii

Evaluate x to three decimal places.

a

3^{x} = 11

b

11^{x} = 3

c

7^{x} = \dfrac{1}{13}

d

343^{x} = 12\,167

e

7^{ - x } = 21

f

4^{x - 2} = 10

g

4^{ 2 x - 8} = 70

6

For each of the following incorrect sets of working:

i

Which step was incorrect? Explain your answer.

ii

Rearrange the original equation into the form a = \dfrac{\log A}{\log B}.

iii

Evaluate a to three decimal places.

a

\begin{aligned} 9 ^ {a} &= 40 \\ \log9^{a} &= \log40 & (1)\\ a + \log 9 &= \log40 & (2)\\ a &= \log40 - \log9 & (3)\\ &\approx 0.648 & (4) \end{aligned}

b

\begin{aligned} 2 ^ {a} &= 89 \\ \log2^{a} &= \log89 & (1) \\ a \log 2 &= \log89 & (2)\\ a &= \log_{89} 2 & (3)\\ &\approx 0.154 & (4) \end{aligned}

7

Solve for y in each of the following equations, correct to two decimal places:

a

8^{y} = 15

b

2^{y + 1} = 11

c

3^{y + \left( - 5 \right)} = 11^{y}

d

5^{y + \left( - 4 \right)} = 6^{y + 3}

8

Solve for the exact value of x in each of the following equations:

a

2 \left(11^{x}\right) = 4

b

\dfrac{1}{3} \left(11^{x}\right) = 2

c

2^{ 3 x} = 3^{x - 1}

d

5^{ 5 x} = 3^{ 2 x + 3}

e

5^{x} = 2^{ 4 x + 6}

f

2^{ 3 x} = 3^{ 7 x - 1}

g

2^{ 3 x + 4} = 5^{x - 1}

h

2^{ 2 x - 7} = 7^{ 3 x + 5}

i

5 \left(2^{x}\right) = 7 - 2^{x + 3}

j

\left(7^{x}\right)^{2} - 4 \left(7^{x}\right) = 0

k

\left(5^{x}\right)^{2} + 2 \left(5^{x}\right) - 24 = 0

l

\left(3^{x}\right)^{2} - 3 \left(3^{x}\right) = 10

9

Solve for x in each of the following equations, correct to two decimal places:

a

2^{6 - x} = \dfrac{1}{14}

b

3^{x} + 3 = 84

c

\left(\dfrac{1}{6}\right)^{x + 3} = \sqrt{7}

d

7^{x} = 3^{x + 1}

e

3 \left(5^{ 2 x - 1}\right) + 1 = 5

f

2 \log_{10}^{2} x - \log_{10} x = 6

g

\log_{10} x = x^{2} - 12 x + 8

h

2^{x} = 3 - 7^{x + 1}

i

2^{x} = \log_{10} \left(x + 5\right)

j

x = 6^{x}

k

\left(\log_{5} x\right)^{2} - \log_{5} \left(x^{6}\right) + 9 = 0

l

3 x + 2 = 4^{x}

10

The equation for the population at time t is given by Q = 30 \times 8^{ 5 t}. Make t the subject of the equation.

11

If A = 5 \log y - 50, make y the subject of the equation.

12

Rearrange the following equations to make M the subject:

a

u = 5 \log \left(\dfrac{M}{N}\right)

b

m = 2.1 \log \left(\dfrac{M}{N}\right) + 8

13

Given that \log p = \log q + r \log x, form an equation in which p is the subject, and no logarithms are involved.

14

If A = P \left(\dfrac{1 - \left(1 + r\right)^{ - t }}{r}\right), make t the subject of the equation.

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Outcomes

U34.AoS2.4

solution of equations of the form f(x) = g(x) over a specified interval, where f and g are functions of the type specified in the ‘Functions, relations and graphs’ area of study, by graphical, numerical and algebraic methods, as applicable

U34.AoS2.11

apply algebraic, logarithmic and circular function properties to the simplification of expressions and the solution of equations

U34.AoS2.8

analytical, graphical and numerical approaches to solving equations and the nature of corresponding solutions (real, exact or approximate) and the effect of domain restrictions

U34.AoS2.9

apply a range of analytical, graphical and numerical processes (including the algorithm for Newton’s method), as appropriate, to obtain general and specific solutions (exact or approximate) to equations (including literal equations) over a given domain and be able to verify solutions to a particular equation or equations over a given domain

U34.AoS2.7

exponent laws and logarithm laws

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