Solve the Logarithmic Inequality: log₁/₂(5) - log₁/₂(4) ≤ log₁/₂(x) - log₁/₂(3)

Logarithmic Inequalities with Base Less Than One

x=? x=\text{?}

log125log124log12xlog123 \log_{\frac{1}{2}}5-\log_{\frac{1}{2}}4\le\log_{\frac{1}{2}}x-\log_{\frac{1}{2}}3

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:05 We'll use the logarithmic subtraction formula, we'll get the log of the numerator
00:17 Let's equate the logarithms
00:27 Let's isolate X
00:37 This is the appropriate domain
00:42 Let's check the domain of definition and find the solution
00:45 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

x=? x=\text{?}

log125log124log12xlog123 \log_{\frac{1}{2}}5-\log_{\frac{1}{2}}4\le\log_{\frac{1}{2}}x-\log_{\frac{1}{2}}3

2

Step-by-step solution

To solve the inequality involving logarithms with base 12\frac{1}{2}, we will perform the following steps:

  • Step 1: Apply the subtraction property of logarithms.
  • Step 2: Simplify and solve the inequality.

Let's go through the steps:

Step 1: Simplify both sides using the logarithm subtraction rule:

Left side: log125log124=log12(54)\log_{\frac{1}{2}}5 - \log_{\frac{1}{2}}4 = \log_{\frac{1}{2}}\left(\frac{5}{4}\right)

Right side: log12xlog123=log12(x3)\log_{\frac{1}{2}}x - \log_{\frac{1}{2}}3 = \log_{\frac{1}{2}}\left(\frac{x}{3}\right)

This gives us the inequality:

log12(54)log12(x3)\log_{\frac{1}{2}}\left(\frac{5}{4}\right) \le \log_{\frac{1}{2}}\left(\frac{x}{3}\right)

Step 2: Since 12\frac{1}{2} is less than 1, the inequality sign flips when we remove the logarithms.

This gives: 54x3\frac{5}{4} \ge \frac{x}{3}

Multiplying both sides by 3 to solve for xx:

354x3 \cdot \frac{5}{4} \ge x

154x\frac{15}{4} \ge x

Thus, x154x \le \frac{15}{4}, which simplifies to x3.75x \le 3.75.

Since we assumed x>0x > 0, the final solution is:

0<x3.750 < x \le 3.75

3

Final Answer

0<x3.75 0 < x\le3.75

Key Points to Remember

Essential concepts to master this topic
  • Property Rule: Use logablogac=loga(bc) \log_a b - \log_a c = \log_a \left(\frac{b}{c}\right) to combine logarithms
  • Base Analysis: When base < 1, inequality flips: log1/2Alog1/2B \log_{1/2} A \le \log_{1/2} B becomes AB A \ge B
  • Domain Check: Verify x > 0 and substitute back: x=3 x = 3 gives equal sides ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to flip inequality sign with fractional base
    Don't keep the same inequality direction when base < 1 = wrong solution range! When the logarithm base is between 0 and 1, the logarithm function is decreasing, so inequalities reverse. Always flip the inequality sign when removing logarithms with fractional bases.

Practice Quiz

Test your knowledge with interactive questions

\( \log_{10}3+\log_{10}4= \)

FAQ

Everything you need to know about this question

Why does the inequality sign flip when the base is 1/2?

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Because 12<1 \frac{1}{2} < 1 , the logarithm function is decreasing. This means larger inputs give smaller outputs, so inequalities reverse when you remove the logarithm.

How do I know when to apply the logarithm subtraction rule?

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Use logablogac=loga(bc) \log_a b - \log_a c = \log_a \left(\frac{b}{c}\right) whenever you see subtraction between logarithms with the same base. This simplifies the inequality significantly!

What happens if I forget about the domain restriction x > 0?

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Logarithms are only defined for positive arguments. Without x > 0, your solution might include invalid values that make the original equation undefined.

Can I solve this without combining the logarithms first?

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While possible, it's much harder! Combining logarithms first using the subtraction property gives you a cleaner inequality that's easier to solve and understand.

How do I check my final answer?

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Substitute a test value from your solution range back into the original inequality. For example, try x=2 x = 2 : both sides should satisfy the inequality relationship.

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