Solve Logarithmic Equation: 1/2 log₃(x⁴) = log₃(3x² + 5x + 1)

Logarithmic Equations with Power Properties

12log3(x4)=log3(3x2+5x+1) \frac{1}{2}\log_3(x^4)=\log_3(3x^2+5x+1)

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
00:05 Use the logarithm formula for exponents, and move the 0.5 into the logarithm
00:17 Raising to the power of one-half is like taking the square root
00:27 Break down X⁴ into X squared squared
00:32 Square root cancels square
00:34 Substitute in our exercise
00:42 Compare the numbers
00:52 Group terms and arrange the equation
00:57 Use the root formula to find possible solutions
01:12 There are always 2 possible solutions, addition and subtraction
01:27 Find the domain
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

12log3(x4)=log3(3x2+5x+1) \frac{1}{2}\log_3(x^4)=\log_3(3x^2+5x+1)

x=? x=\text{?}

2

Step-by-step solution

To solve the equation 12log3(x4)=log3(3x2+5x+1) \frac{1}{2}\log_3(x^4) = \log_3(3x^2 + 5x + 1) , we will first use the power property of logarithms.

  • Step 1: Apply the power property to the left side: 12log3(x4)=log3(x4)12=log3(x2) \frac{1}{2}\log_3(x^4) = \log_3(x^4)^{\frac{1}{2}} = \log_3(x^2) .

  • Step 2: Now, equating the arguments on both sides, we have: x2=3x2+5x+1 x^2 = 3x^2 + 5x + 1 .

  • Step 3: Rearrange the equation to form a standard quadratic: 0=2x2+5x+1 0 = 2x^2 + 5x + 1 or 2x2+5x+1=0 2x^2 + 5x + 1 = 0 .

  • Step 4: Solve the quadratic using the quadratic formula: x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} , where a=2 a = 2 , b=5 b = 5 , and c=1 c = 1 .

  • Step 5: Substitute the coefficients into the quadratic formula:

  • x=5±5242122=5±2584=5±174 \begin{aligned} x &= \frac{-5 \pm \sqrt{5^2 - 4 \cdot 2 \cdot 1}}{2 \cdot 2} \\ &= \frac{-5 \pm \sqrt{25 - 8}}{4} \\ &= \frac{-5 \pm \sqrt{17}}{4} \end{aligned}

Since we need the solutions to keep the arguments of the logarithms positive, we ensure that 3x2+5x+1>0 3x^2 + 5x + 1 > 0 for values of x x from our solution set.

Thus, the solutions satisfying these conditions are given by x=54±174 x = -\frac{5}{4} \pm \frac{\sqrt{17}}{4} . Therefore, the correct answer is choice 1: 54±174 -\frac{5}{4} \pm \frac{\sqrt{17}}{4} .

3

Final Answer

54±174 -\frac{5}{4}\pm\frac{\sqrt{17}}{4}

Key Points to Remember

Essential concepts to master this topic
  • Power Rule: Convert 12log3(x4) \frac{1}{2}\log_3(x^4) to log3(x2) \log_3(x^2) using exponent properties
  • Equal Arguments: When log3(A)=log3(B) \log_3(A) = \log_3(B) , then A = B directly
  • Domain Check: Verify both x2>0 x^2 > 0 and 3x2+5x+1>0 3x^2 + 5x + 1 > 0 for valid solutions ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to apply power rule to logarithms
    Don't leave 12log3(x4) \frac{1}{2}\log_3(x^4) as is = impossible to solve directly! The coefficient becomes an exponent inside the log. Always convert 12log3(x4) \frac{1}{2}\log_3(x^4) to log3(x2) \log_3(x^2) first.

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 I set the arguments equal when the logarithms are equal?

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When log3(A)=log3(B) \log_3(A) = \log_3(B) , it means 3 raised to the same power gives both A and B. This only happens when A = B exactly!

How do I use the power property of logarithms?

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The rule is clogb(x)=logb(xc) c \cdot \log_b(x) = \log_b(x^c) . So 12log3(x4) \frac{1}{2}\log_3(x^4) becomes log3((x4)1/2)=log3(x2) \log_3((x^4)^{1/2}) = \log_3(x^2) .

Why do I get two solutions from a quadratic?

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Quadratic equations naturally have two solutions unless the discriminant is zero. Here, 2x2+5x+1=0 2x^2 + 5x + 1 = 0 gives us both possible x-values that satisfy the original logarithmic equation.

Do I need to check if the arguments are positive?

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Yes, absolutely! Logarithms are only defined for positive arguments. Check that both x2>0 x^2 > 0 and 3x2+5x+1>0 3x^2 + 5x + 1 > 0 for your solutions.

What if one solution makes the argument negative?

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Then that solution is extraneous and must be rejected! Only keep solutions where all logarithmic arguments remain positive.

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