Solve the Logarithmic Inequality: log₂3 - log₂(x+3) ≤ 8

Logarithmic Inequalities with Exponential Conversion

log23log2(x+3)8 \log_23-\log_2(x+3)\le8

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:07 Let's solve this problem together.
00:12 We'll use the subtraction formula for logarithms. This lets us find the log of a ratio.
00:21 Next, we'll set the logarithms equal to each other.
00:36 Now, let's isolate the variable X.
00:44 And that's how we solve the problem! Great job!

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

log23log2(x+3)8 \log_23-\log_2(x+3)\le8

2

Step-by-step solution

To solve this problem, we'll apply the properties of logarithms and inequality manipulation.

Initially, consider the given inequality:

log23log2(x+3)8 \log_2 3 - \log_2 (x + 3) \le 8

Using the quotient rule of logarithms, combine the logs:

log2(3x+3)8 \log_2 \left(\frac{3}{x + 3}\right) \le 8

The inequality log2(3x+3)8 \log_2 \left(\frac{3}{x + 3}\right) \le 8 can be rewritten by converting the logarithm to an exponential form:

3x+328 \frac{3}{x + 3} \le 2^8

Since 28=256 2^8 = 256 , substitute to get:

3x+3256 \frac{3}{x + 3} \le 256

To remove the fraction, multiply both sides by x+3 x + 3 , assuming x+3>0 x + 3 > 0 to maintain the inequality direction:

3256(x+3) 3 \le 256(x + 3)

Divide by 256 to isolate x+3 x+3 :

3256x+3 \frac{3}{256} \le x + 3

Subtract 3 from both sides to solve for x x :

x32563 x \ge \frac{3}{256} - 3

Given the problem's constraints about the positivity of the logarithm's argument, ensure x>3 x > -3 . Our derived inequality starts from x32563 x \ge \frac{3}{256} - 3 , which satisfies this, thus correctly addressing the domain assumptions.

In conclusion, the solution to the inequality is:

x32563 x \ge \frac{3}{256} - 3

3

Final Answer

x32563 x\ge\frac{3}{256}-3

Key Points to Remember

Essential concepts to master this topic
  • Domain Rule: Argument of logarithm must be positive, so x + 3 > 0
  • Technique: Convert log2(3x+3)8 \log_2 \left(\frac{3}{x + 3}\right) \le 8 to 3x+3256 \frac{3}{x + 3} \le 256
  • Check: Verify x = -2.98... gives 30.02...256 \frac{3}{0.02...} \approx 256 satisfies inequality ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting domain restrictions when solving logarithmic inequalities
    Don't ignore that x + 3 > 0 when multiplying by (x + 3) = wrong inequality direction! This happens because you can't take the log of negative numbers. Always check the domain first and ensure x > -3 throughout your solution.

Practice Quiz

Test your knowledge with interactive questions

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

FAQ

Everything you need to know about this question

Why do I need to use the quotient rule for logarithms first?

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The quotient rule lets you combine log23log2(x+3) \log_2 3 - \log_2(x+3) into log2(3x+3) \log_2 \left(\frac{3}{x+3}\right) . This makes it much easier to convert to exponential form in the next step!

How do I convert from logarithmic to exponential form?

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Remember that logb(y)n \log_b(y) \le n means ybn y \le b^n . So log2(3x+3)8 \log_2 \left(\frac{3}{x+3}\right) \le 8 becomes 3x+328=256 \frac{3}{x+3} \le 2^8 = 256 .

Why can't x equal -3 in this problem?

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If x = -3, then x + 3 = 0, and log2(0) \log_2(0) is undefined! Logarithms only work with positive numbers, so we need x + 3 > 0, meaning x > -3.

When I multiply both sides by (x+3), do I flip the inequality?

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No! Since we established that x + 3 > 0 (from the domain), we're multiplying by a positive number. You only flip inequalities when multiplying or dividing by negative numbers.

How do I simplify the final answer?

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Calculate 32563=3256768256=765256 \frac{3}{256} - 3 = \frac{3}{256} - \frac{768}{256} = \frac{-765}{256} . This is approximately -2.988, which is indeed greater than -3, so our domain is satisfied!

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