Problem 2 - Applied Mathematics: For the Managerial, Life, and Social Sciences

Question

Find the graphical solution of each inequality. $$3 y+2>0$$

Step-by-Step Solution

Answer

Isolate y in the inequality: \(y > -\frac{2}{3}\). The boundary line is \(y = -\frac{2}{3}\) which is a horizontal line. Graph this with a dashed line. The test point (0,0) makes the inequality true, so shade the region above the boundary line. This shaded region represents the graphical solution of the inequality \(3y + 2 > 0\).

Step 1: Make y the subject of the inequality.

First, we have to isolate y by solving for it: \(3y + 2 > 0\) Subtract 2 from both sides: \(3y > -2\) Divide by 3: \(y > \frac{-2}{3}\)

Step 2: Identify the boundary line.

Now, we need to find the boundary line by using the equation with an equality sign: \(y = \frac{-2}{3}\) The boundary line is a horizontal line crossing the y-axis at \(-\frac{2}{3}\).

Step 3: Graph the line of the equation.

Plot the line \(y = -\frac{2}{3}\) on the graph. Since the inequality is "greater than," we will use a dashed line to represent the boundary line.

Step 4: Determine the region where the inequality is true.

To determine the region where the inequality is true (\(y > \frac{-2}{3}\)), we need to use a test point. A convenient test point is the origin (0,0). Plug in the coordinates of the origin into the inequality: \(0 > \frac{-2}{3}\) As this statement is true, it means that the region where the inequality is true includes the origin (0,0).

Step 5: Shade the region representing the graphical solution.

Shade the region above the boundary line (as the inequality says y is greater than -2/3), including the origin (0,0). This shaded region represents the graphical solution of the inequality \(3y + 2 > 0\).

Key Concepts

Solving inequalities graphically

When it comes to understanding how to solve inequalities, the graphical method offers a visual representation that can be incredibly intuitive. Simply put, solving an inequality graphically involves translating the inequality from a numerical statement into a picture on a coordinate grid.

Firstly, you'll need to manipulate the inequality to solve for one variable—typically y in the case of two-dimensional graphs. Through this method, inequalities such as \(3y + 2 > 0\) become \(y > -\frac{2}{3}\), which is more straightforward to graph. We represent y-values on a vertical axis, each corresponding to a potential solution to the inequality. By plotting these solutions, we obtain a region in the graph that satisfies the inequality, giving us our graphical solution.

Boundary line in inequalities

Understanding the concept of a boundary line is crucial when solving inequalities graphically. A boundary line divides the coordinate plane into two regions: one that satisfies the inequality and one that does not.

In our example, after isolating y, we get \(y = -\frac{2}{3}\), which represents the boundary line. It's important to note that this line can be solid or dashed. A solid line means that the points on the line satisfy the inequality (\(\leq\) or \(\geq\)), while a dashed line means that they do not (\(<\) or \(>\)). Here, as our inequality is strict (\(y > -\frac{2}{3}\)), we use a dashed line to show that points on the line aren't included in the solution set.

Shading regions in inequalities

Once we have our boundary line drawn, it's time to determine which side of the line represents the solution to our inequality. This is where shading comes in.

To illustrate this concept, think of the boundary line as the edge of a shadow. For our inequality \(y > -\frac{2}{3}\), we shade the region above the boundary line. This signifies that all points in that upper region make the inequality true. Conversely, if our inequality had been \(y < -\frac{2}{3}\), we would have shaded the region below the line. Shading makes the graphical solution easily recognizable at a glance, highlighting the set of points that satisfy the inequality.

Test point method

The test point method is a reliable way to determine which region to shade when solving inequalities graphically.

Select a point that is not on the boundary line—typically, the origin (0,0) is a convenient and common choice. Then, plug this point into the original inequality. If the inequality holds true with this test point, then the region including that point is part of the solution set. In our example, when the origin is substituted into the inequality \(y > -\frac{2}{3}\), we get \(0 > -\frac{2}{3}\), which is a true statement. Therefore, the region containing the origin is shaded. This technique confirms that we've shaded the correct region on our graph.