Graph Solutions to Quadratic Inequalities :

In this section, we will learn, how to graph solutions to quadratic inequalities.

Let f(x) = ax² + bx + c, be a quadratic function or expression. a, b, c ∈ R, a ≠ 0

Then f(x) ≥ 0, f(x) > 0, f(x) ≤ 0 and f(x) < 0 are known as quadratic inequalities.

Graph Solutions to Quadratic Inequalities - Working Rule

Step 1:

Factorize the quadratic expression and obtain its solution by equating the linear factors to zero.

Step 2 :

Plot the roots obtained on real line. The roots will divide the real line in three parts.

Step 3 :

In the right most part, the quadratic expression will have positive sign and in the left most part, the expression will have positive sign and in the middle part, the expression will have negative sign.

Step 4 :

Obtain the solution set of the given inequality by selecting the appropriate part.

Step 5 :

If the inequality contains equality operator (i.e. ≤, ≥), include the roots in the solution set.

Graph Solutions to Quadratic Inequalities - Examples

Example 1 :

Graph and solve for x :

x² - 7x + 6 > 0

Solution :

x² - 7x + 6 > 0

(x - 1)(x - 6) > 0

On equating the factors to zero, we see that x = 1, x = 6 are the roots of the quadratic equation. Plotting these roots on real line and marking positive and negative alternatively from the right most part we obtain the corresponding number line as

If we plot these points on the number line, we will get intervals (-∞, 1) (1, 6) (6, ∞).

 From (-∞, 1) let us take -1 (-1 − 1) (-1 − 6) > 0-2(-7) > 014 > 0 True From (1, 6) let us take 4 (4 − 1) (4 − 6) > 03(-2) > 0-6 > 0 False From (6, ∞) let us take 7 (7 − 1) (7 − 6) > 06(1) > 06 > 0 True

Hence, the solution set is

(− ∞, 1) ∪ (6, ∞)

Example 2 :

Graph and solve for x :

-x² + 3x - 2 > 0

Solution :

-x² + 3x - 2 > 0

Multiplying by negative sign on both sides

x²- 3x + 2 < 0

(x − 1) (x − 2) < 0

x - 1 = 0    x - 2 = 0

x = 1 and x = 2

On equating the factors to zero, we see that x = 1, x = 2 are the roots of the quadratic equation. Plotting these roots on real line and marking positive and negative alternatively from the right most part we obtain the corresponding number line as

If we plot these points on the number line, we will get intervals (-∞, 1) (1, 2) (2, ∞).

 From (-∞, 1) let us take -1 (x − 1) (x − 2) < 0 (-1 − 1) (-1 − 2) < 0(-2)(-3) > 06 < 0 False From (1, 2) let us take 1.5 (x − 1) (x − 2) < 0 (1.5 − 1) (1.5 − 2) < 0(-0.5)(-0.5) > 00.25 < 0 True From (2, ∞) let us take 7 (7 − 1) (7 − 6) > 06(1) > 06 > 0 False

Hence, the solution set is

(1, 2)

Example 3 :

Graph and solve for x :

4x² - 25      0

Solution :

4x² - 25      0

(2x)² - 5²    0

(2x - 5) (2x + 5)     0

2x - 5     0  (or)    2x + 5     0

x =  5/2   (or)   x  =   -5/2

On equating the factors to zero, we see that x = 5/2, x = -5/2 are the roots of the quadratic equation. Plotting these roots on real line and marking positive and negative alternatively from the right most part we obtain the corresponding number line as

If we plot these points on the number line, we will get intervals (-∞, -5/2) (-5/2, 5/2) (5/2, ∞).Graph solutions to quadratic inequalities

 From (-∞, -5/2) let us take -3 (2x - 5) (2x + 5)  ≥   0  (-6 − 5) (-6 + 5) < 0(-11)(-1) > 011 > 0 True From (-5/2, 5/2) let us take 0 (2x - 5) (2x + 5)  ≥   0  (0− 5) (0 + 5) < 0(-5)(5) > 0-25 > 0 False From (5/2, ∞) let us take 4 (2x - 5) (2x + 5)  ≥   0  (8 − 5) (8 + 5) < 0(3)(13) > 039 > 0 True

Hence, the solution set is

(-∞, -5/2) U (5/2, ∞)

After having gone through the stuff given above, we hope that the students would have understood, how to graph solutions to quadratic inequalities.

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