*gre.al.9 GRE Exam Inequality with modulus or absolute value

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Discussion Overview

The discussion revolves around solving the inequality involving absolute values, specifically the expression $|y+3| \le 4$. Participants explore the implications of the inequality, considering different cases based on the sign of the expression inside the absolute value. The scope includes mathematical reasoning and technical explanations related to inequalities and absolute values.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant derives the interval $-7 \le y \le 1$ by considering both cases of the absolute value definition.
  • Another participant presents a similar conclusion using a different approach, stating $|y+3| \le 4 \implies -7 \le y \le 1$.
  • Some participants question whether the initial assumptions about the sign of $y$ are valid, suggesting that the derived interval may imply certain conditions about the signs of $y$.
  • A participant provides a formal definition of absolute value and outlines the cases for solving the inequality, reinforcing the previous conclusions.

Areas of Agreement / Disagreement

While there is agreement on the derived interval $-7 \le y \le 1$, there is contention regarding the assumptions made about the sign of $y$. Some participants express uncertainty about whether the solutions assume $y$ is positive.

Contextual Notes

Participants have not fully resolved the implications of the derived interval on the signs of $y$, and there are differing interpretations of the assumptions involved in the solution process.

karush
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given
$|y+3|\le 4$
we don't know if y is plus or negative so
$y+3\le 4 \Rightarrow y\le 1$
and
$-(y+3)\le 4$
reverse the inequality
$ y+3 \ge -4$
then isolate y
$y \ge -7$
the interval is
$-7 \le y \le 1$
 
Last edited:
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$|y+3| \le 4 \implies -4 \le y+3 \le 4 \implies -7 \le y \le 1$
 
That was quick..
Doesn't that assume y is positive
 
karush said:
That was quick..
Doesn't that assume y is positive

what does the inequality, ${\color{red}-7 \le y} \le 1$, tell you about the possible signs for $y$?

also, see attached graph ...
 

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definition of absolute value ...

$|\text{whatever}| = \left\{\begin{matrix}
\text{whatever}, & \text{if whatever}\ge 0\\
-(\text{whatever}), & \text{if whatever}< 0
\end{matrix}\right.$

therefore ...

$|y+3| = \left\{\begin{matrix}
y+3 \, , &\text{if }y+3 \ge 0 \\
-(y+3) & \text{if }y+3<0
\end{matrix}\right.$

$|y+3| \le 4$

case 1, $y+3 \ge 0$

$y+3 \le 4 \implies y \le 1$

case 2, $y+3 < 0$

$-(y+3) \le 4 \implies y+3 \ge -4 \implies y \ge -7$
 

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