Part of ALG-09 — Quadratic Inequalities & Modulus Functions

Quadratic Inequality — Standard Method

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To solve ax2+bx+cax^{2+bx+c} > 0 (or <, >=, <=): (1) Find discriminant D = b24acb^{2-4ac}. (2) If D>0, find roots alpha < beta. (3) For a>0: parabola opens upward → positive outside roots, negative between. For a<0: reversed. (4) Include endpoints for >= or <=. (5) If D=0: expression touches x-axis at one point, sign doesn't change except at that point. (6) If D<0: sign is same as 'a' everywhere. Always sketch the parabola mentally.

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