Problem 21 - College Algebra

Question

In Exercises 1–26, graph each inequality. $$y \geq x^{2}-9$$

Step-by-Step Solution

Answer

The graph of the inequality \(y \geq x^{2}-9\) is a parabola opening upwards, and the region above (including the boundary) has been shaded.

Step 1: Identify quadratic function

Recognize that \(x^{2}-9\) is a quadratic function. Quadratic functions produce a parabolic graph, opening upwards if the coefficient of \(x^{2}\) is positive which is the case here.

Step 2: Graph the boundary line

Next, graph the boundary line, which is \(y=x^{2}-9\). This forms a parabola that opens upward with the vertex at (0,-9) and intercepts at (-3, 0) and (3, 0).

Step 3: Shade the region

Since the inequality includes 'greater than or equal to', that means all y-values that are greater than or equal to \(x^{2}-9\) must be included. So, shade the region of the graph above (including) the parabola. This is the set of solutions for the inequality.

Key Concepts

Understanding Quadratic Functions

Quadratic functions are mathematical expressions of the form f(x) = ax^{2} + bx + c, where a, b, and c are constants with a ≠ 0. They play a crucial role in algebra and are recognized for the parabolic shapes they exhibit on a graph.

The standard form allows us to identify key features of a parabola, such as its direction of opening, vertex, and axis of symmetry. For example, if a > 0, the parabola opens upwards, and if a < 0, it opens downwards. The vertex of the parabola is the highest or lowest point on the graph, depending on whether it opens downward or upward, respectively.

Furthermore, quadratic functions are used to formulate a range of real-world problems, such as the trajectory of a projectile, the area of a rectangle with given perimeter, and many others. Understanding the properties of quadratic functions is essential for solving related mathematical problems and interpreting their graphical representations accurately.

Graphing a Parabola

A parabola is the graphical representation of a quadratic function and has a distinctive 'U' or inverted 'U' shape, known as concave up or concave down, respectively.

When graphing a parabola, you'll often start by finding the vertex, which is a point where the parabola changes direction. You can find the x-coordinate of the vertex using the formula -b/(2a), and then plug that back into the quadratic function to find the y-coordinate. This point is essential as it serves as a guide for the symmetrical nature of the parabola.

Next, by identifying points on either side of the vertex and using symmetry, you can draw a smooth curve to complete the parabola. Remember that the coefficient of the x^{2} term will tell you if the parabola opens up (positive coefficient) or down (negative coefficient). Additionally, the y-intercept (when x=0) and potential x-intercepts (when y=0) help shape the precise graph of the parabolic curve.

Solving Quadratic Inequalities

Quadratic inequalities are expressions involving a quadratic function set within an inequality, such as y ≶ ax^{2} + bx + c.

To solve these inequalities and graph them, first, graph the related parabola described by the equivalent quadratic equation y = ax^{2} + bx + c. This curve acts as the boundary between different regions on the graph.

Depending on whether the inequality is 'greater than' or 'less than', you will shade either above or below this boundary. Including the boundary line when you have 'greater than or equal to' or 'less than or equal to' is important, which can be represented by a solid line on the graph. The solution of a quadratic inequality is not a single point, but rather a region on the graph where the inequality holds true.

It helps to test points within the regions to ensure they satisfy the original inequality. This way, you can confirm that the shaded region on the graph represents all the possible solutions to the quadratic inequality.