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

Question

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

?=x

Video Solution

Solution Steps

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 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 .

Answer

1 1


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