Teacher resources and professional development across the curriculum

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Insights Into Algebra 1 - Teaching For Learning
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Workshop 4 Quadratic Functions Topic Overview
Topic Overview:

Quadratic functions
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Quadratic functions

Quadratic functions model a number of real-world situations. They describe the path of a ball in flight, represent a cross section of a headlight's reflector, and enable business people to forecast sales.

Mathematical Definition
Role in the Curriculum
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A quadratic function is a second degree equation - that is, 2 is the highest power of the independent variable. Written in standard form, the equation y = ax2 + bx + c (a 0) represents quadratic functions.

When graphed in the coordinate plane, a quadratic function takes the shape of a parabola. To see a parabola in the real world, throw a ball. The path the ball traces as it travels through the air - in an arc to its highest point, then back to the ground in a similar arc - is a parabola. Naturally, the ball bounces after hitting the ground, and each time it does so, it traces another parabola.

In another example, suppose a builder wants to build a parking lot that is rectangular in shape and measures 250 feet around three of the four sides. Write an equation that models the area of the parking lot as a function of the width of the parking lot. Find the dimensions of a parking lot that will enclose the greatest area. If the width of the parking lot is x, then the length of the parking lot is 250 - 2x. So, the area of the parking lot can be modeled by A = (250-2x)x. A graph of the function shows that the maximum area occurs at the vertex of the parabola. This point is located at (62.5, 7812.5). This means that the width of the parking lot with the greatest area is 62.5 feet, the length is 125 feet, and the area is 7812.5 square feet.

Other examples:
  • During freefall, the distance d (in feet) that an object falls is represented by the quadratic function d = 16t2, where t is the time (in seconds) that the object has fallen.

  • Quadratic functions describe the relationship between height (from the ground) and time (in seconds) of a ball as it bounces.

  • A rectangle with a border of 100 feet has perimeter 2l + 2w = 100, which can be rewritten as:

    The quadratic function that represents the area:

Mathematical Definition

A quadratic function is an equation of the form y = ax2 + bx + c (a 0). Its graph is a parabola.

Another widely accepted definition is: A quadratic polynomial is a polynomial of the second degree - that is, a polynomial of the form ax2 + bx + c. A quadratic function is a function f whose value f(x) at x is given by a quadratic polynomial. If f(x) = ax2 + bx + c, then the graph of f is the graph of the equation y = ax2 + bx + c and is a parabola with vertical axes.
(Source: James, Robert C. and Glenn James. Mathematics Dictionary (4th edition). New York: Chapman & Hall, 1976.)

Role in the Curriculum

In Algebra 1, students explore linear functions in great detail, and they learn to graph lines by using tables and plotting points. They can use the same skills to explore quadratic functions. Indeed, it is most appropriate for students to first graph quadratic functions by making a table of x and y values and plotting points. This way, in addition to discovering the parabolic shape of quadratic equations, they can practice using previously acquired skills.

But quadratic equations provide more than just an opportunity for embedded review of graphing. The exploration of functions - particularly quadratic functions and cubic functions - allows students to investigate important topics and explore mathematical patterns. The National Council of Teachers of Mathematics (NCTM) recommends in Principles and Standards for School Mathematics (PSSM):

Students should have substantial experience in exploring the properties of different classes of functions. For instance, they should learn that the function f(x) = x2 - 2x - 3 is quadratic, that its graph is a parabola, and that the graph opens "up" because the leading coefficient is positive. They should also learn that some quadratic equations do not have real roots and that this characteristic corresponds to the fact that their graphs do not cross the x-axis. And they should be able to identify the complex roots of such quadratics.
In addition to identifying properties of a parabola from the standard form (y = ax2 + bx + c), it is also important for students to recognize the properties of a parabola in the vertex form (y = a (x - h)2 + k). Of these two forms, the vertex form provides more valuable information to assist in graphing the function.

In vertex form, the point (h, k) is the vertex of the parabola, and the value of a determines the vertical stretch or shrink of the parabola. A positive value of a will make the parabola open upward, while a negative value of a will make the parabola open downward.

Understanding why and how the transformations alter the graph is important. Students need to understand the effects of a, h, and k in the equation y = a (x - h)2 + k.

To understand the effects of k, students can compare the functions y = x2 and y = x2 +3. The table below shows the pattern of values.

By looking at both the table of values and the graph, students will begin to understand that k produces a vertical shift; in this case, k = 3 shifts the parabola up 3 units. In general, the graph is shifted k units up if k is positive, and k units down if k is negative.

To understand the effects of h, students can compare the functions y = x2 and y = (x + 3)2. The table below shows the patterns of values.

By looking at both the table of values and the graph, students will begin to understand that h produces a horizontal shift; in this case, h = -3 shifts the parabola 3 units to the left. In general, the graph is shifted h units to the right if h is positive, and h units to the left if h is negative. Expressing the equation in vertex form, y = (x + 3)2 = (x- (-3))2.

To understand the effects of a, students can compare the functions y = x2 and y = 3x2. The table below shows a pattern of values.

By looking at both the table of values and the graph, students will begin to understand that a produces a vertical stretch or shrink of the parabola; in this case, a = 3 stretches the parabola vertically by a factor of 3 and makes it appear "skinnier". In general, the parabola is stretched by a factor of a. In addition, if the value of a is negative, the parabola will open downward instead of upward.

Because the standard and vertex forms of a quadratic function reveal different pieces of information, it is important for students to recognize both and be able to convert one to the other. While symbolic manipulation receives less attention today than in the past, it is a necessary skill for converting a quadratic function from the standard form to the vertex form.

As its name implies, the vertex form is more useful when it's important to know the vertex. When a salesman wants to know the maximum profit, or when a rocket scientist needs to know the maximum height of a projectile, the vertex form is preferable. However, when it's more important to know the x-intercepts - for instance, when a pilot wishes to determine the point at which a package dropped from a helicopter will land - the standard form is preferable, because the well-known quadratic formula can be used to determine the roots (or solutions) of the equation.

(The quadratic formula is

where a, b, and c represent the constants in y = ax2 + bx + c.)

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