Solve log₄(9x) + log₄(x+4) - log₄(3) = ln(2e) + ln(1/2e): Finding X

Logarithmic Equations with Mixed Bases

log49x+log4(x+4)log43=ln2e+ln12e \log_49x+\log_4(x+4)-\log_43=\ln2e+\ln\frac{1}{2e}

Find X

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find X
00:06 We'll use the formula for logical addition, we'll get the log of their product
00:16 We'll use this formula in our exercise
00:41 We'll use the formula for logical subtraction, we'll get the log of their quotient
00:46 We'll use this formula in our exercise
00:54 Let's simplify what we can
01:01 We'll use the formula to convert from ln to log
01:20 We'll use this formula in our exercise
01:33 Let's simplify as much as possible
01:43 Open parentheses properly, multiply by each factor
01:53 Let's arrange the equation
02:18 We'll use the root formula to find possible solutions
02:33 Let's calculate and solve
03:38 Let's find the domain
03:55 This is the complete domain
04:02 Always remember there are 2 possibilities, addition and subtraction
04:06 This is the domain, with it we'll find the solution
04:14 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

log49x+log4(x+4)log43=ln2e+ln12e \log_49x+\log_4(x+4)-\log_43=\ln2e+\ln\frac{1}{2e}

Find X

2

Step-by-step solution

To solve this logarithmic equation, we will simplify both sides using logarithm properties.

Step 1: Combine the logarithms on the left side.

The left side is log49x+log4(x+4)log43 \log_4 9x + \log_4 (x+4) - \log_4 3 . Using the properties of logarithms, we can combine these logs:

log4(9x(x+4)3)\log_4 \left( \frac{9x(x+4)}{3} \right)

This simplifies to:

log4(3x(x+4))\log_4 \left(3x(x+4)\right)

Step 2: Simplify the right side.

The right side is ln2e+ln12e \ln 2e + \ln \frac{1}{2e} . Using properties of natural logarithms, combine as follows:

ln(2e12e)=ln1=0\ln \left(2e \cdot \frac{1}{2e}\right) = \ln 1 = 0

Step 3: Equating both sides, we have:

log4(3x(x+4))=0\log_4 \left(3x(x+4)\right) = 0

Step 4: Convert the logarithmic equation to an exponential equation. Since the logarithmic expression equals zero, it signifies:

3x(x+4)=40=13x(x+4) = 4^0 = 1

Step 5: Solve the equation 3x(x+4)=13x(x+4) = 1:

Combine and expand the terms:

3x2+12x1=03x^2 + 12x - 1 = 0

Step 6: Solve the quadratic equation using the quadratic formula x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}, where a=3a = 3, b=12b = 12, and c=1c = -1:

x=12±1224×3×(1)2×3x = \frac{-12 \pm \sqrt{12^2 - 4 \times 3 \times (-1)}}{2 \times 3}

Calculate:

x=12±144+126x = \frac{-12 \pm \sqrt{144 + 12}}{6}

x=12±1566x = \frac{-12 \pm \sqrt{156}}{6}

x=12±4×396x = \frac{-12 \pm \sqrt{4 \times 39}}{6}

x=12±2396x = \frac{-12 \pm 2\sqrt{39}}{6}

x=6±393x = \frac{-6 \pm \sqrt{39}}{3}

Thus, the solution is:

x=2+393x = -2 + \frac{\sqrt{39}}{3}

This matches the correct choice.

Therefore, the solution to the problem is 2+393-2+\frac{\sqrt{39}}{3}.

3

Final Answer

2+393 -2+\frac{\sqrt{39}}{3}

Key Points to Remember

Essential concepts to master this topic
  • Logarithm Properties: Combine logs using addition/subtraction rules before solving
  • Technique: ln(2e)+ln(12e)=ln(1)=0 \ln(2e) + \ln(\frac{1}{2e}) = \ln(1) = 0
  • Check: Substitute x=2+393 x = -2 + \frac{\sqrt{39}}{3} back into original equation ✓

Common Mistakes

Avoid these frequent errors
  • Solving logarithms without combining terms first
    Don't try to solve each log term separately = complicated mess! This makes the equation much harder and often leads to incorrect solutions. Always combine logarithms using properties first, then convert to exponential form.

Practice Quiz

Test your knowledge with interactive questions

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

FAQ

Everything you need to know about this question

Why does ln(2e) + ln(1/2e) equal 0?

+

Using the logarithm property lna+lnb=ln(ab) \ln a + \ln b = \ln(ab) , we get ln(2e12e)=ln(1)=0 \ln(2e \cdot \frac{1}{2e}) = \ln(1) = 0 . The 2e terms cancel out perfectly!

How do I combine logarithms with different operations?

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Use these key properties: logax+logay=loga(xy) \log_a x + \log_a y = \log_a(xy) and logaxlogay=loga(xy) \log_a x - \log_a y = \log_a(\frac{x}{y}) . Apply them step by step from left to right.

Why do I only use the positive solution?

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We need to check which solutions make the original logarithms defined. Since log4(9x) \log_4(9x) requires x>0 x > 0 , we must verify that our solution is positive and satisfies all domain restrictions.

What if I get confused with the quadratic formula?

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Break it down step by step! For 3x2+12x1=0 3x^2 + 12x - 1 = 0 : a=3,b=12,c=1 a = 3, b = 12, c = -1 . Then carefully calculate b24ac=144+12=156 b^2 - 4ac = 144 + 12 = 156 .

How do I simplify √156?

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Factor out perfect squares: 156=4×39=239 \sqrt{156} = \sqrt{4 \times 39} = 2\sqrt{39} . Always look for perfect square factors to simplify radicals!

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