Solve |x+1| ≤ 7-x: Absolute Value Inequality Analysis

Absolute Value Inequalities with Case Analysis

Given:

x+17x \left|x + 1\right| \leq 7 - x

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:

x+17x \left|x + 1\right| \leq 7 - x

Which of the following statements is necessarily true?

2

Step-by-step solution

To solve x+17x \left|x + 1\right| \leq 7 - x , we split it into two cases due to the absolute value:

1. x+17x x + 1 \leq 7 - x

2. x17x -x - 1 \leq 7 - x

Solving case 1:

x+17xx+x712x6x3 x + 1 \leq 7 - x \Rightarrow x + x \leq 7 - 1 \Rightarrow 2x \leq 6 \Rightarrow x \leq 3

Solving case 2:

x17x17 -x - 1 \leq 7 - x \Rightarrow -1 \leq 7 (always true)

Both parts confirm that x3 x \leq 3 is the only true inequality.

3

Final Answer

x3 x \leq 3

Key Points to Remember

Essential concepts to master this topic
  • Definition: x+17x |x+1| \leq 7-x means distance from -1 is at most 7x 7-x
  • Technique: Split into cases: x+10 x+1 \geq 0 and x+1<0 x+1 < 0 for different expressions
  • Check: Test x=0 x = 0 : 0+1=170=7 |0+1| = 1 \leq 7-0 = 7

Common Mistakes

Avoid these frequent errors
  • Solving absolute value inequality without considering domain restrictions
    Don't ignore that 7x0 7-x \geq 0 is required for the inequality to make sense = missing solutions or including invalid ones! Absolute value inequalities AB |A| \leq B require B0 B \geq 0 . Always check that 7x0 7-x \geq 0 , so x7 x \leq 7 .

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 split the absolute value into two cases?

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The expression inside x+1 |x+1| can be positive or negative. When x+10 x+1 \geq 0 , we get x+1=x+1 |x+1| = x+1 . When x+1<0 x+1 < 0 , we get x+1=(x+1) |x+1| = -(x+1) .

What does the right side 7x 7-x represent?

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The right side 7x 7-x represents the maximum allowed distance from -1. Since distances can't be negative, we need 7x0 7-x \geq 0 , which means x7 x \leq 7 .

Why is the second case always true?

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In case 2, when x<1 x < -1 , we get x17x -x-1 \leq 7-x . Adding x x to both sides gives 17 -1 \leq 7 , which is always true regardless of x x value.

How do I combine the results from both cases?

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Case 1 gives x3 x \leq 3 when x1 x \geq -1 . Case 2 is always satisfied when x<1 x < -1 . But we also need x7 x \leq 7 for the inequality to make sense. The intersection gives us x3 x \leq 3 .

Can I check my answer by plugging in test values?

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Absolutely! Try x=0 x = 0 : 0+1=1 |0+1| = 1 and 70=7 7-0 = 7 , so 17 1 \leq 7 ✓. Try x=4 x = 4 : 4+1=5 |4+1| = 5 and 74=3 7-4 = 3 , so 53 5 \leq 3 ✗.

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