Solve the Logarithmic Equation: log₄(x²+8x+1)/log₄8 = 2

Logarithmic Equations with Quadratic Formula

log4(x2+8x+1)log48=2 \frac{\log_4(x^2+8x+1)}{\log_48}=2

x=? x=\text{?}

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find X
00:04 Use the formula for dividing logarithms
00:11 Get the logarithm of the numerator with base of denominator
00:15 Use this formula in our exercise
00:25 Solve the logarithm to find X
00:30 Arrange the equation
00:45 Use the root formula to find possible solutions
00:55 Calculate and solve
01:20 There will always be 2 solutions, addition and subtraction
01:26 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

log4(x2+8x+1)log48=2 \frac{\log_4(x^2+8x+1)}{\log_48}=2

x=? x=\text{?}

2

Step-by-step solution

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

  • Step 1: Simplify the given expression using logarithmic identities.
  • Step 2: Solve the resulting quadratic equation for x x .

Now, let's work through each step:

Step 1: We start with the equation:

log4(x2+8x+1)log48=2 \frac{\log_4(x^2+8x+1)}{\log_4 8} = 2

We know that log48=32 \log_4 8 = \frac{3}{2} , since 8=43/2 8 = 4^{3/2} . Thus, we can rewrite the equation as:

log4(x2+8x+1)=2×32=3 \log_4(x^2+8x+1) = 2 \times \frac{3}{2} = 3

Applying the property of logarithms that states logba=ca=bc \log_b a = c \Rightarrow a = b^c , we have:

x2+8x+1=43=64 x^2 + 8x + 1 = 4^3 = 64

Step 2: Solve the resulting quadratic equation:

x2+8x+1=64 x^2 + 8x + 1 = 64

Subtract 64 from both sides to bring the equation to standard form:

x2+8x+164=0 x^2 + 8x + 1 - 64 = 0

x2+8x63=0 x^2 + 8x - 63 = 0

Now, apply the quadratic formula, x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} , where a=1 a = 1 , b=8 b = 8 , and c=63 c = -63 :

x=8±8241(63)21 x = \frac{-8 \pm \sqrt{8^2 - 4 \cdot 1 \cdot (-63)}}{2 \cdot 1}

x=8±64+2522 x = \frac{-8 \pm \sqrt{64 + 252}}{2}

x=8±3162 x = \frac{-8 \pm \sqrt{316}}{2}

Simplify 316 \sqrt{316} as 794=279 \sqrt{79 \cdot 4} = 2\sqrt{79} :

x=8±2792 x = \frac{-8 \pm 2\sqrt{79}}{2}

Thus, x=4±79 x = -4 \pm \sqrt{79} .

Therefore, the solution to the equation is x=4±79 x = -4 \pm \sqrt{79} .

3

Final Answer

4±79 -4\pm\sqrt{79}

Key Points to Remember

Essential concepts to master this topic
  • Logarithmic Identity: First simplify the denominator using log properties
  • Technique: Convert log48=32 \log_4 8 = \frac{3}{2} since 8=43/2 8 = 4^{3/2}
  • Check: Substitute solutions back: both x=4±79 x = -4 ± \sqrt{79} satisfy original equation ✓

Common Mistakes

Avoid these frequent errors
  • Not simplifying log₄8 first
    Don't leave log48 \log_4 8 unsimplified = complicated fractions! This makes the equation much harder to solve. Always convert log48=32 \log_4 8 = \frac{3}{2} first by recognizing that 8=43/2 8 = 4^{3/2} .

Practice Quiz

Test your knowledge with interactive questions

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

FAQ

Everything you need to know about this question

How do I know that log₄8 equals 3/2?

+

Think: what power of 4 gives us 8? Since 43/2=(43)1/2=641/2=8 4^{3/2} = (4^3)^{1/2} = 64^{1/2} = 8 , we have log48=32 \log_4 8 = \frac{3}{2} .

Why do we get two solutions for x?

+

Because we end up with a quadratic equation x2+8x63=0 x^2 + 8x - 63 = 0 . Quadratic equations typically have two real solutions unless the discriminant is negative or zero.

Do both solutions work in the original equation?

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Yes! Both x=4+79 x = -4 + \sqrt{79} and x=479 x = -4 - \sqrt{79} make x2+8x+1>0 x^2 + 8x + 1 > 0 , so the logarithm is defined.

What if I can't simplify √316?

+

Look for perfect square factors! 316=4×79=4×79=279 \sqrt{316} = \sqrt{4 \times 79} = \sqrt{4} \times \sqrt{79} = 2\sqrt{79} . This helps simplify the final answer.

Can I solve this without the quadratic formula?

+

You could try factoring x2+8x63=0 x^2 + 8x - 63 = 0 , but since 63 doesn't factor nicely with a sum of 8, the quadratic formula is your best option here.

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