Solve Absolute Value Inequality: |2x + 3| > 4x + 5

Absolute Value Inequalities with Two Cases

Given:

2x+3>4x+5 \left|2x + 3\right| > 4x + 5

Which of the following statements is necessarily true?

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Step-by-step written solution

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1

Understand the problem

Given:

2x+3>4x+5 \left|2x + 3\right| > 4x + 5

Which of the following statements is necessarily true?

2

Step-by-step solution

We start with the inequality: 2x+3>4x+5 \left|2x + 3\right| > 4x + 5

This absolute value inequality breaks into two separate inequalities:

  • 2x+3>4x+5 2x + 3 > 4x + 5

  • 2x+3<(4x+5) 2x + 3 < -(4x + 5)

Solving the first inequality: 2x+3>4x+5 2x + 3 > 4x + 5

  • Subtract 2x 2x from both sides: 3>2x+5 3 > 2x + 5

  • Subtract 5 5 from both sides: 2>2x -2 > 2x

  • Divide by 2 2 : x<1 x < -1

Solving the second inequality: 2x+3<(4x+5) 2x + 3 < -(4x + 5)

  • Distribute the negative: 2x+3<4x5 2x + 3 < -4x - 5

  • Add 4x 4x to both sides: 6x+3<5 6x + 3 < -5

  • Subtract 3 3 from both sides: 6x<8 6x < -8

  • Divide by 6 6 : x<43 x < -\frac{4}{3}

The solution is the intersection: x<1 x < -1 .

3

Final Answer

x<1 x < -1

Key Points to Remember

Essential concepts to master this topic
  • Rule: Split |expression| > value into two separate inequalities
  • Technique: Solve 2x + 3 > 4x + 5 and 2x + 3 < -(4x + 5)
  • Check: Test x = -2: |-1| > -3 gives 1 > -3 ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to flip the inequality sign in the second case
    Don't just write 2x + 3 < 4x + 5 for the second case = missing negative solutions! The absolute value definition requires the expression to be less than the NEGATIVE of the right side. Always write 2x + 3 < -(4x + 5) for the second inequality.

Practice Quiz

Test your knowledge with interactive questions

Given:

\( \left|2x-1\right|>-10 \)

Which of the following statements is necessarily true?

FAQ

Everything you need to know about this question

Why do I need to consider two separate cases?

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The absolute value |expression| represents distance from zero, which can be positive or negative inside. So |2x + 3| > 4x + 5 means either 2x + 3 is positive and greater than 4x + 5, or 2x + 3 is negative but its absolute value is still greater.

How do I know which case applies?

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You solve both cases and then find where they overlap! The final answer is the intersection of both solutions, not their union.

What if I get different x-values from each case?

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That's normal! You need to find the intersection of both solution sets. In this problem, x < -4/3 AND x < -1, so the answer is x < -1 (the more restrictive condition).

Can absolute value inequalities have no solution?

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Yes! If the intersection of both cases is empty, or if the right side is negative (since absolute values are never negative), then there's no solution.

How do I check my answer?

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Pick a value from your solution set and substitute it back. For x = -2: 2(2)+3=1=1 |2(-2) + 3| = |-1| = 1 and 4(2)+5=3 4(-2) + 5 = -3 . Since 1 > -3, it works! ✓

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