Simultaneous Logarithmic Equations System

00:00 9 marks

  • Exam Style - Paper 3
  • HL
  • Tier 3
  • Simultaneous equations
  • Domain restrictions
  • Logarithmic equations

Solve the following system of logarithmic equations:

\(\log(x) + \log(y) = 2\)
\(\log(x) – \log(y) = 1\)

Your task is to:

(a) Set up the equivalent exponential equations using logarithmic properties.
(b) Solve the system for \(x\) and \(y\).
(c) Validate that the solutions satisfy the domain restrictions for the logarithm.

Part a

4 minutes 3 marks

Question

Starting with the system:

\(\log(x) + \log(y) = 2\)
\(\log(x) – \log(y) = 1\)

Use logarithm properties to rewrite each equation in exponential form.



Hint Hint

Hint

Combine the logarithms using the properties \(\log(a) + \log(b) = \log(ab)\) and \(\log(a) – \log(b) = \log(\frac{a}{b})\). Remember that \(\log(x)\) implies base 10 logarithm.

  • Answer Answer

Complete Answer

The equations become \(xy = 10^2 = 100\) and \(\frac{x}{y} = 10^1 = 10\).

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Part b

4 minutes 3 marks

Question

Solve the system of equations obtained in part (a):

\(xy = 100\)
\(\frac{x}{y} = 10\)



Hint Hint

Hint

From the second equation, express \(x\) in terms of \(y\) and substitute this expression into the first equation to solve for \(y\).

  • Answer Answer

Complete Answer

The solution is \(x = 10\sqrt{10}\) and \(y = \sqrt{10}\).

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Part c

4 minutes 3 marks

Question

Validate the solution by ensuring that it meets the domain restrictions for logarithmic functions.



Hint Hint

Hint

For \(\log(x)\) and \(\log(y)\) to be defined, both \(x\) and \(y\) must be strictly greater than zero. Check if the obtained values of \(x\) and \(y\) satisfy these conditions.

  • Answer Answer

Complete Answer

Since \(x = 10\sqrt{10} > 0\) and \(y = \sqrt{10} > 0\), the solution is valid.

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