RevisionLinear Inequations

Algebraic and Graphical Solutions.

By I Porter

IntroductionAn inequation is formed when two mathematical statements have an unequalitysign between them.Comon inequality signs:

> is greater than

< is less than

≥ is greater than or equal to

≤ is less than or equal to

Inequations can have an infinite number of solutions.

Solving inequations makes use of the following axioms of inequality for realnumbers a, b and c.

If a > b , then

1. a + c > b + c

2. a - c > b - c

3. ac > bc if c > 0

4. if c > 0

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ac

>bc

5. ac < bc if c < 0

6. if c < 0

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ac

<bc

Similar axioms also apply for a < b.

Solving Inequalities

Inequations may be simplified by:

1. adding the same number to both sides.

2. subtracting the same number to both sides.

3. multiplying both sides by the same positive number.

4. dividing both sides by the same positive number.

i.e. 10 > 3, then 10 + 2 > 3 + 2

i.e. 10 > 3, then 10 - 2 > 3 - 2

i.e. 10 > 3, then 10 x 2 > 3 x 2

i.e. 10 > 3, then

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102

>32

In all cases above, the direction of the inequality remains the same.Also, the above statements apply when ‘>’ is replaced with ‘<‘.

Special Cases

The inequality sign must be reversed when:

1. multiplying both sides by the same negative number.

2. dividing both sides by the same negative number.

i.e. 10 > 3, but 10 x -2 < 3 x -2

i.e. 10 > 3, then <

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10−2

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3−2

16 17 18 19 20x

-12 -11 -10 -9 -8x

-7 -6 -5 -4 -3x

Graphical Solutions

Algebra Graphical Number Line Solution

x > 4

x < -5

x ≥ -10

x ≤ 18

Note: ‘>’ and ‘< ‘ use and open circle.

Note: ‘≥’ and ‘≤‘ use and closed (dot) circle.

You also only need to write in three (3) numbers to indicate location and order.

4 5 632x

x

ExamplesInequations are solve exactly the same way as equations, with two exception as stated by the axioms (5) and (6). [ reverse the inequality when x or by a negative number ]

Solve and graph a solution for the following:

a) 4x - 5 < 23

4x < 28

x < 7

876x

b) 4(2 - x) ≥ 3x + 14

8 - 4x ≥ 3x + 14

8 - 7x ≥ 14

- 7x ≥ 6

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x ≤−67

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−67

€

−57

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−1

Add 5 to both sides.

Divide both sides by 4.

Open circle, arrow left.

Expand Brackets.

Subtract 3x from both sides.

Subtract 8 from both sides.

Divide both sides by -7.

Reverse inequality sign.Closed circle, arrow left.

Always move ALGEBRA to the LEFT SIDE

Examples: Solve and graph a solution for the following:

a)

€

x −15

≤x +1

3

€

3(x −1) ≤ 5(x +1)

€

3x − 3 ≤ 5x + 5

€

−2x − 3 ≤ 5

€

−2x ≤ 8

€

x ≥ −4

-3-4-5x

Cross multiply denominators.

Expand brackets.

Subtract 5x from both sides.

Add 3 to both sides.

Divide both sides by -2.

Reverse inequality sign.Closed circle, arrow right.

Always move ALGEBRA to the LEFT SIDE

b)

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−5 < 2x − 3 ≤ 7

€

−2 < 2x ≤10

€

−1< x ≤ 5

Add 3 to both sides.

Divide both sides by 2.

-2 -1 0 1 2 3 4 5 6 7

Exercise: Solve and graph a solution for each the following:

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1) 2x + 5 < −5

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2) 7x < 3(2x +1)

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3) 2x + 7 < 3x +10

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4) x − 5

2>

5x − 36

€

6) − 3 <2x −1

3≤ 3

€

5) 22 ≤ 5x − 3 < 32

€

x < −5

€

x < 3

€

x > −3

€

x < −6

€

5 ≤ x < 7

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−4 < x ≤ 5

-4-5-6x

432x

-2-3-4x

-5-6-7x 6 7 854 x 0 5-4 x