Solve Log Equation: log7x + log(x+1) - log7 = log2x - logx

Logarithmic Equations with Domain Restrictions

log7x+log(x+1)log7=log2xlogx \log7x+\log(x+1)-\log7=\log2x-\log x

?=x ?=x

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:07 Find the domain
00:22 This is the domain
00:40 We'll use the formula for logical addition, we'll get the log of their product
00:47 We'll use the formula for logical subtraction, we'll get the log of their quotient
01:06 We'll use these formulas in our exercise, and we'll get this log
01:36 Simplify what we can
02:01 Isolate X
02:16 We'll use the trinomial to find possible solutions
02:36 Check the domain
02:40 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

log7x+log(x+1)log7=log2xlogx \log7x+\log(x+1)-\log7=\log2x-\log x

?=x ?=x

2

Step-by-step solution

Defined domain

x>0 x>0

x+1>0 x+1>0

x>1 x>-1

log7x+log(x+1)log7=log2xlogx \log7x+\log\left(x+1\right)-\log7=\log2x-\log x

log7x(x+1)7=log2xx \log\frac{7x\cdot\left(x+1\right)}{7}=\log\frac{2x}{x}

We reduce by: 7 7 and by X X

x(x+1)=2 x\left(x+1\right)=2

x2+x2=0 x^2+x-2=0

(x+2)(x1)=0 \left(x+2\right)\left(x-1\right)=0

x+2=0 x+2=0

x=2 x=-2

Undefined domain x>0 x>0

x1=0 x-1=0

x=1 x=1

Defined domain

3

Final Answer

1 1

Key Points to Remember

Essential concepts to master this topic
  • Domain: All arguments of logarithms must be positive
  • Technique: Use log properties to combine: loga+logb=log(ab) \log a + \log b = \log(ab)
  • Check: Verify solutions satisfy domain x>0 x > 0 and equation ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to check domain restrictions after solving
    Don't accept x = -2 just because it satisfies the quadratic equation = invalid solution! Negative values make logarithms undefined. Always verify each solution satisfies all domain conditions from the original equation.

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 x be negative in logarithmic equations?

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Logarithms are only defined for positive arguments. When you have log(x) \log(x) or log(7x) \log(7x) , the value inside must be greater than zero for the logarithm to exist!

How do I combine multiple logarithms in one equation?

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Use these key properties:

  • loga+logb=log(ab) \log a + \log b = \log(ab)
  • logalogb=log(ab) \log a - \log b = \log(\frac{a}{b})

Apply them step by step to simplify both sides.

What if I get multiple solutions from the quadratic?

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Test each solution individually against the original domain restrictions. Only solutions that make all logarithmic arguments positive are valid answers.

Can I cancel out the logarithms directly?

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Yes! Once you have logA=logB \log A = \log B , you can conclude that A = B. This gives you a simpler equation without logarithms to solve.

Why do we need x > 0 AND x > -1?

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We need both conditions because we have multiple logarithmic terms. Since log(7x) \log(7x) requires x > 0 and log(x+1) \log(x+1) requires x > -1, we take the stricter condition: x > 0.

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