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Section 1.1

Real Numbers and Inequalities

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Key rules

  • A number is rational if it can be written as pq\frac{p}{q} with integers p,qp, q and q≠0q \neq 0. Terminating and repeating decimals are rational; non-terminating, non-repeating decimals (like 5\sqrt{5}, π\pi) are irrational.
  • Multiplying or dividing an inequality by a negative number reverses the inequality sign.
  • Interval notation: [a,b][a,b] includes endpoints, (a,b)(a,b) excludes them, and ∞\infty always gets a round bracket.
  • ∣x∣<a  ⟺  −a<x<a|x| < a \iff -a < x < a, and ∣x∣>a  ⟺  x<−a|x| > a \iff x < -a or x>ax > a (for a>0a > 0). Also a2=∣a∣\sqrt{a^2} = |a|.
  • For quadratic and rational inequalities, move everything to one side, find where the expression is zero or undefined, and use a sign table.

Visual guide

Number lines for the intervals (−1, 4], [2, ∞) and (−3, 3). Open circles mark endpoints that are not included, closed circles mark endpoints that are included.(−1, 4]−1 < x ≤ 4−5−4−3−2−1012345[2, ∞)x ≥ 2−5−4−3−2−1012345(−3, 3)|x| < 3−5−4−3−2−1012345
Interval notation on a number line. An open circle means the endpoint is not included, a closed circle means it is. Infinity always gets a round bracket.

Questions

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