Solve: log₁/₃(e²ln x) < 3log₁/₃(2) Logarithmic Inequality

Logarithmic Inequalities with Base Conversion

log13e2lnx<3log132 \log_{\frac{1}{3}}e^2\ln x<3\log_{\frac{1}{3}}2

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:07 Convert between logs
00:17 Use the logarithm product formula, switch between bases
00:27 Use the power property of logarithms, raise the number by the coefficient
00:38 Again use the formula and extract the power
00:48 Use the power property of logarithms, raise the number by the coefficient
00:53 Compare the logarithm numbers
01:13 Isolate X
01:18 Find the appropriate domain
01:23 Check the domain of definition and find the solution
01:33 And this is the solution to the problem

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

log13e2lnx<3log132 \log_{\frac{1}{3}}e^2\ln x<3\log_{\frac{1}{3}}2

2

Step-by-step solution

To solve this problem, we'll follow these key steps:

  • Separate the components inside the logarithm using the property: logb(ac)=logb(a)+logb(c)\log_b(a \cdot c) = \log_b(a) + \log_b(c).
  • Apply the power property: logb(ac)=clogb(a)\log_b(a^c) = c\log_b(a).
  • Simplify the inequality and solve it.

Consider the inequality given:

log13(e2lnx)<3log13(2) \log_{\frac{1}{3}}(e^2\ln x) < 3\log_{\frac{1}{3}}(2)

Using the product property of logarithms, we can rewrite this as:

log13(e2)+log13(lnx)<3log13(2) \log_{\frac{1}{3}}(e^2) + \log_{\frac{1}{3}}(\ln x) < 3\log_{\frac{1}{3}}(2)

Next, apply the power property to simplify log13(e2)\log_{\frac{1}{3}}(e^2):

2log13(e)+log13(lnx)<3log13(2) 2\log_{\frac{1}{3}}(e) + \log_{\frac{1}{3}}(\ln x) < 3\log_{\frac{1}{3}}(2)

Let a=log13(e) a = \log_{\frac{1}{3}}(e) and b=log13(2) b = \log_{\frac{1}{3}}(2) . The inequality becomes:

2a+log13(lnx)<3b 2a + \log_{\frac{1}{3}}(\ln x) < 3b

Rearrange to isolate log13(lnx)\log_{\frac{1}{3}}(\ln x):

log13(lnx)<3b2a \log_{\frac{1}{3}}(\ln x) < 3b - 2a

Since 13\frac{1}{3} is less than 1, meaning the inequality reverses when converting back to exponential form:

lnx>(13)(3b2a) \ln x > \left(\frac{1}{3}\right)^{(3b - 2a)}

Converting the expression on the right-hand side to exponential form:

lnx>(13)log13(8) \ln x > (\frac{1}{3})^{\log_{\frac{1}{3}}(8)}

This simplifies to:

lnx>18 \ln x > \frac{1}{8}

Take the exponential of both sides to solve for xx:

x>e18 x > e^{\frac{1}{8}}

Simplifying gives:

x>8 x > \sqrt{8}

Therefore, the solution to the problem is 8<x \sqrt{8} < x .

3

Final Answer

8<x \sqrt{8} < x

Key Points to Remember

Essential concepts to master this topic
  • Property: Use logb(ac)=logb(a)+logb(c) \log_b(ac) = \log_b(a) + \log_b(c) to separate products
  • Technique: Apply power rule: log1/3(e2)=2log1/3(e) \log_{1/3}(e^2) = 2\log_{1/3}(e)
  • Check: When base < 1, inequality flips: x>8 x > \sqrt{8} means x>22 x > 2\sqrt{2}

Common Mistakes

Avoid these frequent errors
  • Forgetting to flip inequality sign with fractional base
    Don't keep the same inequality direction when base is 1/3 < 1 = wrong solution! When the base is between 0 and 1, logarithmic functions are decreasing, so inequalities reverse. Always flip the inequality sign when converting from logarithmic to exponential form with fractional bases.

Practice Quiz

Test your knowledge with interactive questions

\( \log_75-\log_72= \)

FAQ

Everything you need to know about this question

Why does the inequality sign flip when the base is 1/3?

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When the base is between 0 and 1 (like 1/3), the logarithmic function is decreasing. This means larger inputs give smaller outputs, so when you convert back to exponential form, the inequality direction must reverse!

How do I simplify log₁/₃(e²)?

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Use the power rule: log1/3(e2)=2log1/3(e) \log_{1/3}(e^2) = 2\log_{1/3}(e) . The exponent 2 comes down as a coefficient, making the expression easier to work with.

What does 3log₁/₃(2) equal exactly?

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This equals log1/3(23)=log1/3(8) \log_{1/3}(2^3) = \log_{1/3}(8) . The coefficient 3 becomes an exponent on the argument using the power property in reverse.

Why is the final answer x > √8 instead of a range?

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Since we need lnx>18 \ln x > \frac{1}{8} , and ln x is only defined for positive x, we automatically have x > 0. Combined with our inequality, we get x>8 x > \sqrt{8} .

How can I check if x = 3 satisfies the original inequality?

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Substitute x = 3: log1/3(e2ln3) \log_{1/3}(e^2 \ln 3) vs 3log1/3(2) 3\log_{1/3}(2) . Since 3 > √8 ≈ 2.83, it should satisfy the inequality. Calculate both sides to verify!

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