Solve Logarithmic Inequality: log₁/₄(9) vs log₅(7)/log₅(1/4)

Logarithmic Inequalities with Change of Base

Is inequality true?

log149<log57log514 \log_{\frac{1}{4}}9<\frac{\log_57}{\log_5\frac{1}{4}}

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Step-by-step video solution

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00:00 Solve
00:04 We'll use the logarithmic subtraction formula, we'll get log divided by
00:14 We'll use this formula in our exercise
00:19 And this is the solution to the question

Step-by-step written solution

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1

Understand the problem

Is inequality true?

log149<log57log514 \log_{\frac{1}{4}}9<\frac{\log_57}{\log_5\frac{1}{4}}

2

Step-by-step solution

To solve this problem, we will rewrite both sides of the inequality with the change of base formula and evaluate them:

  • Step 1: Rewrite both logarithms using common base.
    Using the change of base formula, log149=log29log214 \log_{\frac{1}{4}}9 = \frac{\log_29}{\log_2\frac{1}{4}} and log57log514\frac{\log_57}{\log_5\frac{1}{4}} can also be rewritten similarly.
  • Step 2: Recognize that changing everything to base 2 will be beneficial.
    log2(1/4)=log241=2\log_2 (1/4) = \log_2 4^{-1} = -2.
  • Step 3: Evaluate left side.
    log149=log292=log292 \log_{\frac{1}{4}}9 = \frac{\log_2 9}{-2} = -\frac{\log_2 9}{2} .
  • Step 4: Evaluate right side using the same base.
    log57log514=log272 \frac{\log_57}{\log_5\frac{1}{4}} = -\frac{\log_2 7}{2} , where log24=2-\log_2 4 = 2.
  • Step 5: Compare both expressions
    log292<log272=>log29<log27=>9<7-\frac{\log_2 9}{2} < -\frac{\log_2 7}{2} => \log_2 9 < \log_2 7 => 9 < 7.
  • Step 6: Since 9>79 > 7, convert the logarithmic expressions back into log14\log_{\frac{1}{4}}.
    log147\log_{\frac{1}{4}}7 is smaller than log149\log_{\frac{1}{4}}9, so inequation holds.

After comparing these expressions, we see that log149<log147 \log_{\frac{1}{4}}9 < \log_{\frac{1}{4}}7 indeed holds true.

Therefore, the solution is: Yes, since: log149<log147 \log_{\frac{1}{4}}9 < \log_{\frac{1}{4}}7 .

3

Final Answer

Yes, since:

log149<log147 \log_{\frac{1}{4}}9<\log_{\frac{1}{4}}7

Key Points to Remember

Essential concepts to master this topic
  • Change of Base: Use logba=logcalogcb \log_b a = \frac{\log_c a}{\log_c b} to convert different bases
  • Technique: Convert log57log514=log147 \frac{\log_5 7}{\log_5 \frac{1}{4}} = \log_{\frac{1}{4}} 7 for direct comparison
  • Check: Since base 14<1 \frac{1}{4} < 1 , larger argument gives smaller log value ✓

Common Mistakes

Avoid these frequent errors
  • Comparing logarithms with different bases directly
    Don't compare log149 \log_{\frac{1}{4}}9 and log57log514 \frac{\log_5 7}{\log_5 \frac{1}{4}} without converting = meaningless comparison! Different bases make direct comparison impossible and lead to wrong conclusions. Always convert to the same base using change of base formula first.

Practice Quiz

Test your knowledge with interactive questions

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

FAQ

Everything you need to know about this question

Why can't I just compare the numbers inside the logarithms?

+

Because different bases change everything! log28=3 \log_2 8 = 3 but log1080.9 \log_{10} 8 \approx 0.9 . You must convert to the same base first using the change of base formula.

How do I use the change of base formula correctly?

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The formula is logba=logcalogcb \log_b a = \frac{\log_c a}{\log_c b} where c is any convenient base. In this problem, log57log514=log147 \frac{\log_5 7}{\log_5 \frac{1}{4}} = \log_{\frac{1}{4}} 7 by the formula.

Why does the inequality direction matter with the base?

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When the base is between 0 and 1 (like 14 \frac{1}{4} ), the logarithm function is decreasing. So if 9>7 9 > 7 , then log149<log147 \log_{\frac{1}{4}} 9 < \log_{\frac{1}{4}} 7 !

How can I remember when logarithm inequalities flip?

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Simple rule: If the base is greater than 1, bigger arguments give bigger logs. If the base is between 0 and 1, bigger arguments give smaller logs. Think of 14 \frac{1}{4} as a 'shrinking' base!

What's the easiest way to check my final answer?

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  • Convert both sides to decimal approximations
  • Use a calculator to verify log1491.58 \log_{\frac{1}{4}} 9 \approx -1.58 and log1471.40 \log_{\frac{1}{4}} 7 \approx -1.40
  • Check that 1.58<1.40 -1.58 < -1.40 is true ✓

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