Solve ln(4x+3) - ln(x²-8) = 2: Finding the Value of x

Logarithmic Equations with Domain Restrictions

ln(4x+3)ln(x28)=2 \ln(4x+3)-\ln(x^2-8)=2

?=x

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:11 We'll use the formula to convert to logarithms
00:38 We'll use the formula for subtracting logarithms, we'll get the logarithm of their ratio
00:52 We'll use these formulas in our exercise
01:04 We'll solve according to the logarithm definition
01:26 We'll multiply by the denominator to eliminate the fraction
01:56 We'll arrange the equation
02:13 We'll use the roots formula to find possible solutions
02:35 We'll calculate and solve
03:41 Remember there are 2 possibilities, addition and subtraction
04:00 These are the possible solutions
04:10 We'll check the domain of definition
05:05 According to the domain of definition, we'll find the solution
05:08 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

ln(4x+3)ln(x28)=2 \ln(4x+3)-\ln(x^2-8)=2

?=x

2

Step-by-step solution

Let's solve the logarithmic equation step-by-step:

Step 1: Combine the Logarithms
Using the property ln(a)ln(b)=ln(ab) \ln(a) - \ln(b) = \ln\left(\frac{a}{b}\right) , we combine the logarithms:

ln(4x+3x28)=2 \ln\left(\frac{4x+3}{x^2-8}\right) = 2

Step 2: Remove the Logarithm by Exponentiation
Exponentiate both sides with base e e to get rid of the natural logarithm:

4x+3x28=e2\frac{4x+3}{x^2-8} = e^2

Step 3: Solve the Resulting Equation
Multiplying both sides by x28 x^2 - 8 to eliminate the fraction:

4x+3=e2(x28) 4x + 3 = e^2(x^2 - 8)

Expanding and rearranging gives us:

e2x24x8e23=0 e^2x^2 - 4x - 8e^2 - 3 = 0

Let's employ the quadratic formula x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} , where a=e2 a = e^2 , b=4 b = -4 , and c=(8e2+3) c = -(8e^2 + 3) .

Calculate the discriminant:

b24ac=(4)24(e2)((8e2+3)) b^2 - 4ac = (-4)^2 - 4(e^2)(-(8e^2 + 3))

Solving this using numerical approximations (since we have e27.39 e^2 \approx 7.39 ), you get:

x3.18 x \approx 3.18

Conclusion:
The value of x x is approximately 3.18 3.18 , which confirms our choice.

3

Final Answer

3.18 3.18

Key Points to Remember

Essential concepts to master this topic
  • Property: ln(a)ln(b)=ln(ab) \ln(a) - \ln(b) = \ln\left(\frac{a}{b}\right) combines logarithms
  • Technique: Exponentiate both sides to get 4x+3x28=e2 \frac{4x+3}{x^2-8} = e^2
  • Check: Verify domain requirements: 4x+3>0 4x+3 > 0 and x28>0 x^2-8 > 0

Common Mistakes

Avoid these frequent errors
  • Forgetting to check domain restrictions after solving
    Don't just solve the equation algebraically without checking if your answer makes the original logarithms defined! If 4x+3 ≤ 0 or x²-8 ≤ 0, the natural logarithm is undefined and your solution is invalid. Always verify that both expressions inside the logarithms are positive.

Practice Quiz

Test your knowledge with interactive questions

\( \log_75-\log_72= \)

FAQ

Everything you need to know about this question

Why can't I have negative values inside the natural logarithm?

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The natural logarithm ln(x) \ln(x) is only defined for positive real numbers. If you try to take ln(5) \ln(-5) or ln(0) \ln(0) , it's mathematically undefined in the real number system.

How do I know which quadratic solution to use?

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After using the quadratic formula, you'll get two potential solutions. Check both by substituting into the domain restrictions. Only keep solutions where both 4x+3>0 4x+3 > 0 and x28>0 x^2-8 > 0 .

What does e² equal exactly?

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e27.389 e^2 \approx 7.389 where e is Euler's number (approximately 2.718). You can use this decimal approximation in your calculations, but keep more precision until your final answer.

Can I solve this without using the logarithm property?

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While you could exponentiate each side separately, using ln(a)ln(b)=ln(ab) \ln(a) - \ln(b) = \ln\left(\frac{a}{b}\right) simplifies the problem significantly. It reduces two logarithms to one, making the algebra much cleaner.

Why does x ≈ 3.18 when the quadratic formula gives exact values?

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The exact answer involves e2 e^2 which is irrational. We use decimal approximations to match the multiple choice format. The exact form would be more complex but equivalent.

What if I get a solution that makes the denominator zero?

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If your solution makes x28=0 x^2-8 = 0 (so x=±22 x = \pm 2\sqrt{2} ), that solution is invalid because it creates division by zero in the original equation. Always exclude such values.

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