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Math I

Quadratic Equations and Inequalities: Practice and Methods

Solve quadratic inequalities by looking at the graph and the xx-axis: the discriminant tells you how many times they meet, and the graph shows where it is above or below.

Basic, Standard: Grade 10 · Term 2 / Advanced: Grade 12 · Exam prep

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The quadratic formula and discriminant

Quadratic formula
ax2+bx+c=0  ⟺  x=−b±b2−4ac2aax^2 + bx + c = 0 \iff x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

If the xx-coefficient is 2b′2b': x=−b′±b′2−acax = \dfrac{-b' \pm \sqrt{b'^2 - ac}}{a}

The sign of the discriminant D=b2−4acD = b^2 - 4ac gives the number of real roots (points where the graph meets the xx-axis): 2 if D>0D > 0, 1 if D=0D = 0 (tangent), and 0 if D<0D < 0.

Solving quadratic inequalities

If a>0a > 0 and ax2+bx+c=0ax^2 + bx + c = 0 has two real roots α<β\alpha < \beta, then

ax2+bx+c>0  ⟺  x<α or x>β,ax2+bx+c<0  ⟺  α<x<βax^2 + bx + c > 0 \iff x < \alpha \text{ or } x > \beta,\qquad ax^2 + bx + c < 0 \iff \alpha < x < \beta
Caution

If the x2x^2 coefficient is negative, multiply by −1-1 first (reversing the inequality). If D≦0D \leqq 0, complete the square: (x−2)2>0(x - 2)^2 > 0 holds for all real numbers except 2, while (x−2)2≦0(x - 2)^2 \leqq 0 only for x=2x = 2.

Inequalities that always hold; location of roots

"x2−2kx+k+6>0x^2 - 2kx + k + 6 > 0 for every real xx" means the upward parabola never meets the xx-axis, so D<0D < 0. If the x2x^2 coefficient contains a parameter, check the case where it is 0 separately.

Location of roots

f(x)=0f(x) = 0 has two distinct positive roots exactly when
① D>0D > 0 ② the axis is positive ③ f(0)>0f(0) > 0.
"One root greater than kk and the other less than kk" is simply f(k)<0f(k) < 0.