Part of ALG-05 — Quadratic Equations

Location of Roots

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Cue Column:

  • Both roots > k?
  • Both roots < k?
  • k between roots?
  • Both roots in interval?

Note Column: For ax2ax^2 + bx + c = 0, a > 0, roots alpha, beta:

  1. Both roots > k: D >= 0 AND f(k) > 0 AND -b2a\frac{b}{2a} > k
  2. Both roots < k: D >= 0 AND f(k) > 0 AND -b2a\frac{b}{2a} < k
  3. k between roots: f(k) < 0 (this alone is sufficient!)
  4. Both roots in (p,q): D >= 0 AND f(p) > 0 AND f(q) > 0 AND p < -b2a\frac{b}{2a} < q
  5. Exactly one root in (p,q): f(p)*f(q) < 0

Summary: Location conditions combine three tools: discriminant, function value at key points, and vertex position. Condition 3 is the simplest — just one inequality.

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