2.7Linear Inequalities and Absolute Value Inequalities

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Chemistry that uses this algebra — the answer to why you are learning it.

Applies1.5Measurement Uncertainty, Accuracy, and Precision
A reported mass of 6.72 g with uncertainty ±0.01 g means the true value satisfies |measured − true| ≤ 0.01, i.e. the value lies in the interval [6.71, 6.73] — every uncertainty figure in this section is silently an absolute-value inequality defining an interval, even though the section never uses interval notation itself.
Applies14.7Acid-Base Titrations
An indicator's usable range is defined as pKa ± 1, and phenolphthalein is stated as colorless for pH < 8.3 and pink for pH > 8.3 — both are literally absolute-value/interval inequalities (|pH − pKa| ≤ 1) used to decide which indicator is suitable for a given titration, not just descriptive language.