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More Lessons for Algebra, Math Worksheets

In this lesson, we will look at solving quadratic inequalities.

Example:

Solve the quadratic inequality x2 – 4x > –3

Solution:

Step 1: Make one side of the inequality zero

x2 – 4x > –3
x2 – 4x + 3 > 0

Step 2: Factor the quadratic expression

x2 – 4x + 3 > 0
(x – 3)(x – 1) > 0

Step 3: Find the range of values of x which satisfies the inequality.
(x – 3)(x – 1) > 0 (y is positive): we choose the interval for which the curve is above the x-axis.
x < 1 or x > 3

Note: If the quadratic inequality was (x – 3)(x – 1) < 0 (y is negative), we would have chosen the interval for which the curve is below the x-axis i.e. 1 < x < 3

Example:

Solve 2x2 < 9x + 5

Solution:

Step 1: Make one side of the inequality zero

2x2 < 9x + 5
2x2 – 9x – 5 < 0

Step 2: Factor the quadratic expression

2x2 – 9x – 5 < 0
(2x + 1)(x – 5) < 0

Step 3: Find the range of values of x which satisfies the inequality.

(2x + 1)(x – 5) < 0 (y is negative): we choose the interval for which the curve is below the x-axis.

Note: If the quadratic inequality was (2x + 1)(x – 5) > 0 (y is positive) we would have chosen the interval for which the curve is above the x-axis i.e. or x > 5

1. Find all the zeros of the polynomial, and arrange the zeros in increasing order. The zeros are called its critical numbers.
2. Plot those numbers on the number line as open or closed points based upon the original inequality symbol.
3. Choose a test value in each interval to see if the interval satisfies the inequality or not. If the test value produces a true statement, the entire interval will be true. If the interval produces a false statement, the entire interval is false.
4. Clearly graph your solution and state the solution using interval notation or inequalities.

Examples:
1. Solve x2 - 6x - 16 ≤ 0
2. Solve 2x2 - 11x + 12 > 0
3. Solve x2 + 4 > 0
Example:
x2 - x - 12 ≤ 0
How to solve a Quadratic Inequality?
Example:
2x2 + 3x - 5 > 0
1) Turn inequality into an equation.
2) Find solutions.
3) Make a number line, and check each solution and interval.
Examples:
Solve x2 + 2x - 8 ≥ 0

Solving Quadratic Inequalities - Step by step
Find all the solutions to
2x2 + 5x - 12 ≥ 0
Find all the solutions to
2x2 < -4x + 6

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