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# Mastering Inequalities: A Comprehensive Guide to Solving Inequalities

## Understanding the Basics of Inequalities

Inequalities are mathematical expressions that compare two values or expressions, indicating that one is greater than, less than, or equal to the other. Inequalities can be represented using symbols such as < (less than), > (greater than), ≤ (less than or equal to), or ≥ (greater than or equal to).

To solve inequalities, it is important to understand the properties of inequalities. For instance, adding or subtracting the same value from both sides of an inequality preserves the inequality, i.e., if a < b, then a + c < b + c for any value of c. Similarly, multiplying or dividing both sides of an inequality by a positive number preserves the inequality, i.e., if a < b, then ac < bc for any positive value of c. However, if we multiply or divide both sides of an inequality by a negative number, the inequality is reversed, i.e., if a < b and c < 0, then ac > bc.

Inequalities can also be solved by graphing the solution set on a number line. For instance, the solution set of an inequality such as x < 3 can be represented by shading the region to the left of 3 on the number line. However, inequalities with absolute values or quadratic expressions may require additional techniques for solving and graphing the solution set.

## Techniques for Solving Linear Inequalities

Linear inequalities are inequalities that involve linear expressions, i.e., expressions of the form ax + b, where a and b are constants and x is a variable. To solve linear inequalities, we follow similar rules as solving linear equations, except that we need to reverse the inequality symbol when we multiply or divide both sides by a negative number.

Here are the steps for solving a linear inequality:

1. Simplify the inequality by combining like terms and moving all the variable terms to one side of the inequality.
2. Divide both sides of the inequality by the coefficient of the variable to isolate the variable term.
3. Reverse the inequality symbol if you multiply or divide both sides of the inequality by a negative number.
4. Check the solution by plugging it back into the original inequality.

For instance, to solve the inequality 2x – 5 ≤ 7, we can follow these steps:

1. Add 5 to both sides of the inequality to get 2x ≤ 12.
2. Divide both sides of the inequality by 2 to get x ≤ 6.
3. The inequality symbol is not reversed since we did not multiply or divide both sides by a negative number.
4. Check the solution by plugging in a value for x, such as x = 4, to get 2(4) – 5 ≤ 7, which is true. Therefore, x ≤ 6 is the solution to the inequality.

## Strategies for Solving Quadratic Inequalities

Quadratic inequalities are inequalities that involve quadratic expressions, i.e., expressions of the form ax^2 + bx + c, where a, b, and c are constants and x is a variable. To solve quadratic inequalities, we first need to find the roots of the quadratic expression by factoring, completing the square, or using the quadratic formula. Then, we use the sign of the quadratic expression to determine the solution set of the inequality.

Here are some strategies for solving quadratic inequalities:

1. Factoring: If the quadratic expression can be factored, we can use the zero product property to find the roots and determine the sign of the quadratic expression. For instance, to solve the inequality x^2 – 4x < 3, we can factor the left-hand side to get (x - 1)(x - 3) < 0. Then, we can use a sign table or graphing to find the solution set, which is x < 1 or x > 3.

2. Completing the square: If the quadratic expression cannot be factored easily, we can complete the square to rewrite it in vertex form and determine the sign of the quadratic expression. For instance, to solve the inequality x^2 + 6x – 7 ≥ 0, we can complete the square by adding (6/2)^2 = 9 to both sides of the inequality to get (x + 3)^2 ≥ 16. Then, we can take the square root of both sides and consider the two cases: x + 3 ≥ 4 or x + 3 ≤ -4. The solution set is x ≥ 1 or x ≤ -7.

3. Quadratic formula: If the quadratic expression cannot be factored or completed by the square method, we can use the quadratic formula to find the roots and determine the sign of the quadratic expression. For instance, to solve the inequality 2x^2 + 5x – 3 < 0, we can use the quadratic formula to get x = (-5 ± √37)/4. Then, we can use a sign table or graphing to find the solution set, which is -3/2 < x < 1/2.

## Solving Inequalities with Absolute Values

Absolute value is a mathematical function that returns the distance of a number from zero, regardless of its sign. The absolute value of a number is always non-negative, i.e., it is greater than or equal to zero. Inequalities that involve absolute values can be solved using different methods, depending on the form of the inequality.

Here are some techniques for solving inequalities with absolute values:

1. Definition of absolute value: If the inequality involves a single absolute value, we can use the definition of absolute value to split the inequality into two cases: one for when the argument of the absolute value is positive and one for when it is negative. For instance, to solve the inequality |x – 3| < 5, we can split it into two cases: x - 3 < 5 or -(x - 3) < 5. Then, we can solve each case separately to get the solution set, which is -2 < x < 8.

2. Graphical approach: If the inequality involves a single absolute value, we can also use a graphical approach to solve it. We can plot the graph of the absolute value function and the equation of the inequality on the same coordinate system and shade the region that satisfies the inequality. For instance, to solve the inequality |2x + 1| > 3, we can plot the graph of y = |2x + 1| and y = 3, and shade the regions above and below the line y = 3. Then, we can find the values of x that are in the shaded regions, which are x < -2 or x > 1.

3. Properties of inequalities: If the inequality involves multiple absolute values, we can use the properties of inequalities to simplify it. For instance, to solve the inequality |x – 2| + |x + 1| < 5, we can use the fact that the sum of two non-negative numbers is greater than or equal to each of the individual numbers. Then, we can simplify the inequality to get -3 < x < 2.

## Graphical Representations of Inequalities and Their Solutions

Graphing inequalities is a useful tool for visualizing the solution set of an inequality. We can graph inequalities on a number line, a coordinate plane, or a 3D space, depending on the number of variables and the form of the inequality. By shading the regions that satisfy the inequality, we can find the solution set of the inequality.

Here are some examples of graphical representations of inequalities and their solutions:

1. Number line: To graph a linear inequality on a number line, we can plot the endpoints of the inequality and shade the region that satisfies the inequality. For instance, to graph the inequality x < 3, we can plot an open circle at 3 and shade the region to the left of 3, since any value of x less than 3 satisfies the inequality.

2. Coordinate plane: To graph a linear inequality with two variables on a coordinate plane, we can plot the boundary line of the inequality and shade the region that satisfies the inequality. For instance, to graph the inequality y > -2x + 3, we can plot the line y = -2x + 3 and shade the region above the line, since any point above the line satisfies the inequality.

3. 3D space: To graph a linear inequality with three variables in a 3D space, we can plot the boundary plane of the inequality and shade the region that satisfies the inequality. For instance, to graph the inequality 2x – y + z < 4, we can plot the plane 2x - y + z = 4 and shade the region below the plane, since any point below the plane satisfies the inequality.

4. System of inequalities: To graph a system of inequalities, we can graph each inequality separately and find the intersection of the shaded regions. For instance, to graph the system of inequalities y > -x + 2 and y < 2x + 3, we can plot the lines y = -x + 2 and y = 2x + 3 and shade the region above the first line and below the second line. The solution set is the intersection of the shaded regions, which is the region between the lines.