Mathematics
Grade 12
15 min
Graph solutions to quadratic inequalities
Graph solutions to quadratic inequalities
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Introduction & Learning Objectives
Learning Objectives
Identify the boundary parabola for a given quadratic inequality.
Determine whether the boundary parabola should be a solid or dashed curve.
Calculate the roots and vertex of the boundary parabola to accurately plot it.
Use a test point to determine the correct region to shade on the coordinate plane.
Graphically represent the complete solution set for a two-variable quadratic inequality.
Interpret the shaded region as the set of all ordered pairs (x, y) that satisfy the inequality.
A rocket's height is modeled by a quadratic function. How can we graph all the moments in time and heights where the rocket is *above* a certain altitude? 🚀
This tutorial builds on your knowledge of graphing parabolas and solving linear inequalities. You will learn how t...
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Key Concepts & Vocabulary
TermDefinitionExample
Quadratic InequalityAn inequality that involves a quadratic expression. In two variables, it takes the form y < ax² + bx + c, y > ax² + bx + c, y ≤ ax² + bx + c, or y ≥ ax² + bx + c.y ≥ x² - 2x - 3 is a quadratic inequality. The solution is the set of all points (x, y) that make the statement true.
Boundary CurveThe parabola y = ax² + bx + c that separates the coordinate plane into two regions: one where the inequality is true and one where it is false.For the inequality y < 2x² + 1, the boundary curve is the parabola y = 2x² + 1.
Solid vs. Dashed BoundaryA solid boundary curve is used for inequalities with ≤ or ≥, indicating that points on the parabola are included in the solution. A dashed boundary curve is used for < or >, indicating that points on...
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Core Formulas
Standard Form of a Quadratic Inequality
y \ [inequality symbol] \ ax^2 + bx + c
Where the inequality symbol can be >, <, ≥, or ≤. This form helps identify the coefficients a, b, and c needed to analyze and graph the boundary parabola.
Quadratic Formula for Roots
x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
Used to find the x-intercepts (roots) of the boundary parabola y = ax² + bx + c. These points are critical for accurately sketching the curve.
Vertex Formula
x_{vertex} = -\frac{b}{2a}
Used to find the x-coordinate of the vertex of the parabola. Substitute this x-value back into the equation y = ax² + bx + c to find the y-coordinate, giving the vertex (h, k).
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Challenging
Consider the system of inequalities: y > x² - 4 and y < -x² + 4. Which statement best describes the solution region?
A.The region above both parabolas.
B.The region below both parabolas.
C.The bounded region between the two parabolas, not including the boundaries.
D.The entire coordinate plane except for the region between the parabolas.
Challenging
The solution to a quadratic inequality is the set of all points outside a dashed parabola with a vertex at (2, 1) and a y-intercept at (0, 5). What is the inequality?
A.y < x² - 4x + 5
B.y > x² - 4x + 5
C.y > -x² + 4x + 5
D.y < (x - 2)² + 1
Challenging
For the inequality y > x² + kx + 9, the solution set contains the entire region where y > 0. What must be true about the constant 'k'?
A.-6 < k < 6
B.k > 6 or k < -6
C.k = 6
D.k < 36
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