Analyze the Inequality: Solving |3x + 9| < 18

Absolute Value Inequalities with Double Bounds

Given:

3x+9<18 \left|3x+9\right|<18

Which of the following statements is necessarily true?

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1

Understand the problem

Given:

3x+9<18 \left|3x+9\right|<18

Which of the following statements is necessarily true?

2

Step-by-step solution

To solve the inequality 3x+9<18\left|3x + 9\right| < 18, follow these steps:

  • Step 1: Remove the absolute value by expressing it as a double inequality:
    18<3x+9<18 -18 < 3x + 9 < 18 .

  • Step 2: Simplify the inequality:
    First, subtract 9 from all parts:
    189<3x+99<189 -18 - 9 < 3x + 9 - 9 < 18 - 9 ,
    which simplifies to 27<3x<9 -27 < 3x < 9 .

  • Step 3: Solve for xx by dividing the entire inequality by 3:
    273<3x3<93 -\frac{27}{3} < \frac{3x}{3} < \frac{9}{3} ,
    resulting in 9<x<3 -9 < x < 3 .

Upon solving, we determine that the solution to the inequality is the interval:

9<x<3 -9 < x < 3 .

3

Final Answer

9<x<3 -9 < x < 3

Key Points to Remember

Essential concepts to master this topic
  • Rule: |A| < B becomes -B < A < B compound inequality
  • Technique: Subtract 9: -18 - 9 < 3x < 18 - 9
  • Check: Test x = 0: |3(0) + 9| = 9 < 18 ✓

Common Mistakes

Avoid these frequent errors
  • Solving as two separate inequalities instead of compound inequality
    Don't split |3x + 9| < 18 into 3x + 9 < 18 OR 3x + 9 > -18 = wrong solution set! This misses the intersection requirement. Always write as one compound inequality: -18 < 3x + 9 < 18.

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 does |3x + 9| < 18 become a compound inequality?

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The absolute value |A| represents distance from zero. When |A| < 18, the expression A must be between -18 and 18, giving us the compound inequality -18 < A < 18.

What's the difference between < and > with absolute values?

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With |A| < B, you get a compound inequality (AND condition): -B < A < B. With |A| > B, you get two separate inequalities (OR condition): A < -B or A > B.

How do I check if my interval is correct?

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Pick any number inside your interval and substitute it back. For 9<x<3 -9 < x < 3 , try x = 0: |3(0) + 9| = 9, and 9 < 18 ✓

Why can't x equal -9 or 3?

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The original inequality uses strict inequality (<). This means the absolute value must be strictly less than 18, not equal to it. So the endpoints are not included.

What if I get confused with the algebra steps?

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Work systematically: Start with -18 < 3x + 9 < 18, then subtract 9 from all three parts, then divide all three parts by 3. Keep the inequality signs pointing the same direction!

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