Solve the Mixed Logarithm Equation: log7×ln(x) = ln7×log(x²+8x-8)

Mixed Logarithm Equations with Domain Restrictions

log7×lnx=ln7log(x2+8x8) \log7\times\ln x=\ln7\cdot\log(x^2+8x-8)

?=x

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:03 We want to find the domain
00:25 Convert to log
00:35 Use the logarithm product formula and switch between bases
00:47 Simplify what we can
00:57 The bases are equal, so we can compare the numbers themselves
01:02 Arrange the equation
01:07 Use the trinomial to find possible solutions
01:20 These are the possible solutions, let's check the domain
01:30 Substitute and check each solution
01:37 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

log7×lnx=ln7log(x2+8x8) \log7\times\ln x=\ln7\cdot\log(x^2+8x-8)

?=x

2

Step-by-step solution

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

  • Step 1: Set up the equation based on the given: log7×lnx=ln7log(x2+8x8)\log7 \times \ln x = \ln7 \cdot \log(x^2 + 8x - 8).
  • Step 2: Utilize logarithmic properties and equate the expressions fully.
  • Step 3: Transform and solve the derived quadratic equation.

Now, let's work through each step:

Step 1: Consider the given equation: log7×lnx=ln7log(x2+8x8)\log7 \times \ln x = \ln7 \cdot \log(x^2 + 8x - 8).

Step 2: We can leverage the commutative property of multiplication to rewrite the equation:
lnxln7=log(x2+8x8)log7\frac{\ln x}{\ln7} = \frac{\log(x^2 + 8x - 8)}{\log7}.

Cross-multiplying gives:
lnxlog7=ln7log(x2+8x8)\ln x \cdot \log7 = \ln7 \cdot \log(x^2 + 8x - 8).

Rule out common denominators to get equality in logs, rewritten equation:
lnx=log(x2+8x8)\ln x = \log(x^2 + 8x - 8).

Step 3: Assume the simplest corresponding argument equality:
x=x2+8x8 x = x^2 + 8x - 8 (consider logarithmic domain; check/simplify where equal in rational space) then solve for real roots / positively defined solutions:

Rearrange to form a quadratic equation:
0=x2+8xx8=x2+7x8 0 = x^2 + 8x - x - 8 = x^2 + 7x - 8

Apply the quadratic formula x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} , where a=1 a = 1 , b=7 b = 7 , c=8 c = -8 :

x=7±49+322 x = \frac{-7 \pm \sqrt{49 + 32}}{2}

x=7±812 x = \frac{-7 \pm \sqrt{81}}{2}

x=7±92 x = \frac{-7 \pm 9}{2}

This results in two possible solutions:
x=22=1andx=162=8 x = \frac{2}{2} = 1 \quad \text{and} \quad x = \frac{-16}{2} = -8

Since logarithms require positive values:
Available within positive domain: x=1 x = 1

Therefore, the solution to the problem is x=1 x = 1 .

3

Final Answer

1 1

Key Points to Remember

Essential concepts to master this topic
  • Base Conversion: Use loga=lnaln10 \log a = \frac{\ln a}{\ln 10} to convert between logarithm types
  • Technique: Cross-multiply then simplify: lnx=log(x2+8x8) \ln x = \log(x^2 + 8x - 8) becomes x=x2+8x8 x = x^2 + 8x - 8
  • Check Domain: Verify x>0 x > 0 and x2+8x8>0 x^2 + 8x - 8 > 0 for valid solutions ✓

Common Mistakes

Avoid these frequent errors
  • Ignoring domain restrictions for logarithms
    Don't accept x = -8 even though it satisfies the quadratic = invalid solution! Logarithms require positive arguments, so ln(x) needs x > 0. Always check that all solutions satisfy the domain requirements of every logarithmic term.

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 use x = -8 as a solution?

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Because logarithms are only defined for positive numbers! Since we need ln(x), we must have x > 0. The value x = -8 makes ln(-8) undefined, so it's not a valid solution.

How do I handle equations with different logarithm bases?

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Use the change of base formula: logab=lnblna \log_a b = \frac{\ln b}{\ln a} . This converts everything to natural logarithms, making the equation easier to solve.

What if I get multiple solutions from the quadratic?

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Always check each solution in the original logarithmic equation! Some solutions might make the arguments of logarithms negative or zero, which means they're extraneous solutions that must be rejected.

Why does the equation simplify to x = x² + 8x - 8?

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After cross-multiplying and using logarithm properties, we assume the arguments are equal when the logarithms are equal. This works when both sides have the same base after conversion.

How do I verify x = 1 is correct?

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  • Left side: log7×ln1=log7×0=0 \log 7 \times \ln 1 = \log 7 \times 0 = 0
  • Right side: ln7×log(1+88)=ln7×log1=ln7×0=0 \ln 7 \times \log(1 + 8 - 8) = \ln 7 \times \log 1 = \ln 7 \times 0 = 0
  • Both sides equal 0, so x = 1 is correct! ✓

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