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

Question

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

?=x

Video Solution

Solution Steps

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 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.

Answer

3.18 3.18


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