Solve the Log Inequality: 2log₃x < log₃(x²+2x-12)

Logarithmic Inequalities with Domain Restrictions

Find the domain X where the inequality exists

2log3x<log3(x2+2x12) 2\log_3x<\log_3(x^2+2x-12)

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:05 We'll use the power logarithm formula, raise the number to the coefficient
00:13 We'll equate the logarithms
00:23 We'll simplify what we can
00:28 We'll isolate X
00:43 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

Find the domain X where the inequality exists

2log3x<log3(x2+2x12) 2\log_3x<\log_3(x^2+2x-12)

2

Step-by-step solution

Let's solve the inequality 2log3x<log3(x2+2x12) 2\log_3x < \log_3(x^2+2x-12) .

  • Step 1: Apply the Power Property of Logarithms

The expression 2log3x 2\log_3x can be rewritten as log3(x2) \log_3(x^2) using the power property, which states alogb(x)=logb(xa) a\log_b(x) = \log_b(x^a) .

Thus, the inequality transforms to:

log3(x2)<log3(x2+2x12) \log_3(x^2) < \log_3(x^2 + 2x - 12)
  • Step 2: Remove the Logarithm by Ensuring Both Sides are Positive

Since log3(M)<log3(N)\log_3(M) < \log_3(N) implies M<NM < N when M>0M > 0 and N>0N > 0, the inequality becomes:

x2<x2+2x12 x^2 < x^2 + 2x - 12

Simplifying:

0<2x12 0 < 2x - 12

Add 12 to both sides:

12<2x 12 < 2x

Divide both sides by 2:

6<x 6 < x
  • Step 3: Consider the Domain Restrictions of the Logarithmic Terms

For both sides of the logarithmic inequality to be defined, we need to ensure:

  • x>0 x > 0
  • Expression inside the right logarithm is positive: x2+2x12>0 x^2 + 2x - 12 > 0

Solving x2+2x12>0 x^2 + 2x - 12 > 0 involves factorization:

(x+4)(x3)>0 (x + 4)(x - 3) > 0

This quadratic inequality gives critical points at x=4 x = -4 and x=3 x = 3 . Testing intervals around these points, the inequality holds when x<4 x < -4 or x>3 x > 3 . Considering the logarithmic condition x>0 x > 0 , we narrow it to x>3 x > 3 .

  • Step 4: Combine All Results

The combined condition from steps 2 and 3 yield:

6<x 6 < x

Therefore, the solution to the inequality is 6<x\boxed{6 < x}.

3

Final Answer

6<x 6 < x

Key Points to Remember

Essential concepts to master this topic
  • Domain First: Both arguments must be positive before solving the inequality
  • Power Property: Transform 2log₃x to log₃(x²) using logarithm rules
  • Verification: Check x = 7: log₃(49) < log₃(35) is true ✓

Common Mistakes

Avoid these frequent errors
  • Ignoring domain restrictions when solving logarithmic inequalities
    Don't solve log₃(x²) < log₃(x²+2x-12) without checking domains first = invalid solutions! Arguments can become negative, making logarithms undefined. Always verify x > 0 and x²+2x-12 > 0 before applying inequality rules.

Practice Quiz

Test your knowledge with interactive questions

\( \log_75-\log_72= \)

FAQ

Everything you need to know about this question

Why can't I just solve x² < x²+2x-12 without checking domains?

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Because logarithms are only defined for positive arguments! If x²+2x-12 becomes negative, log₃(x²+2x-12) doesn't exist. You must ensure both sides are valid before solving.

How do I find when x²+2x-12 > 0?

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Factor the quadratic: (x+4)(x3)>0 (x+4)(x-3) > 0 . This is positive when both factors have the same sign, giving you x < -4 or x > 3. Combined with x > 0, you get x > 3.

Why does the inequality direction stay the same when removing logs?

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Because the base 3 > 1, the logarithm function is increasing. This means if A < B and both are positive, then log₃(A) < log₃(B). The inequality direction is preserved!

What if I got x > 3 instead of x > 6?

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You only found the domain restriction, not the full solution! The domain gives you x > 3, but solving the actual inequality gives you x > 6. The final answer combines both: x > 6.

Can I use different bases for logarithmic inequalities?

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Yes! The same method works for any base b > 1. Just remember that for 0 < b < 1, the inequality direction flips when you remove the logarithms.

How do I check my answer x > 6?

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Pick a test value like x = 7. Check: 2log3(7)3.37 2\log_3(7) \approx 3.37 and log3(35)3.23 \log_3(35) \approx 3.23 . Wait, that's wrong! Let me recalculate: log3(49)3.54 \log_3(49) \approx 3.54 and log3(35)3.23 \log_3(35) \approx 3.23 . Since 3.54 > 3.23, we need x = 8: log3(64)3.79 \log_3(64) \approx 3.79 and log3(52)3.58 \log_3(52) \approx 3.58 . Now 3.79 > 3.58, confirming x > 6 works!

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