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| 17 Theory slides |
| 8 Exercises - Grade E - A |
| Each lesson is meant to take 1-2 classroom sessions |
Here are a few recommended readings before getting started with this lesson.
All three graphs of the functions presented earlier have a very distinctive form. These curves have a specific name, which now will be properly introduced.
A parabola can be vertical or horizontal. A vertical parabola can open upward or downward. In comparison, a horizontal parabola can open to the left or the right.
Equations of parabolas always contain a variable raised to the second power. This is why the functions that represent vertical parabolas are called quadratic functions.
Name | Equation | Characteristics |
---|---|---|
Standard Form | y=ax2+bx+c | a, b, and c are real numbers, a=0, and c is the y-intercept of the parabola. |
Vertex Form | y=a(x−h)2+k | a, h, and k are real numbers, a=0, and (h,k) is the vertex of the parabola. |
Intercept Form (also called Factored Form) |
y=a(x−x1)(x−x2) | a, x1, and x2 are real numbers, a=0, and x1 and x2 are the x-intercepts of the parabola. |
The inherent shape of parabolas gives rise to several characteristics that all quadratic functions have in common.
A parabola either opens upward or downward. This is the direction of the parabola . If the leading coefficient a of the corresponding equation is positive, the parabola opens upward. If the coefficient is negative, the parabola opens downward.
If a function's axis of symmetry is the y-axis, it is said that it is an even function.
The concept of an axis of symmetry extends beyond graphs of functions.
Any geometric figure can have an axis of symmetry if there exists a line that divides it into congruent, mirror-image halves.A parabola can intersect the x-axis at zero, one, or two points. Since the function's value at an x-intercept is always 0, these points are called zeros, or sometimes roots.
Because all graphs of quadratic functions extend infinitely to the left and right, they each have a y-intercept somewhere along the y-axis.
Consider the given parabola. Identify its zeros, line of symmetry, or y-intercept.
A quadratic function is said to be written in vertex form if it has the following format.
y=a(x−h)2+k
Here, a, h, and k are real numbers with a=0. The value of a gives the direction of the parabola. When a>0, the parabola faces upward, and when a<0, it faces downward. The vertex of the parabola lies at (h,k), and the axis of symmetry is the vertical line x=h. Consider the graph of y=-41(x−4)2+8.
Direction | Vertex | Axis of Symmetry |
---|---|---|
a=-41 | h=4 and k=8 | h=4 |
Since -41 is less than 0, the parabola opens downward. | The vertex is located at (4,8). | The axis of symmetry is the vertical line x=4. |
Function 1 | Function 2 | Function 3 |
---|---|---|
y=2(x+1)2+7 ⇕ y=2(x−(-1))2+7 |
y=(x−3)2+1 ⇕ y=1(x−3)2+1 |
y=5(x−2)2−3 ⇕ y=5(x−2)2+(-3) |
When a quadratic function is written in vertex form, some characteristics of its graph can be identified.
y=a(x−h)2+k | ||
---|---|---|
Direction | Vertex | Axis of Symmetry |
a>0 a<0 ⇒ upward⇒ downward
|
(h,k) | x=h |
The axis of symmetry is a vertical line that passes through the vertex and divides the parabola into two mirror images. Since the axis of symmetry is x=h, here it is x=2.
LaShay loves playing golf.
The axis of symmetry of the parabola is the vertical line through the vertex. Therefore, in this case, the equation of the axis of symmetry is x=23.
x=0
Subtract term
(-a)2=a2
(ba)m=bmam
a⋅cb=ca⋅b
ba=b/2a/2
Add fractions
Mark is studying parabolas so that he can help LaShay with her golf swing. He wants to write the vertex form of the quadratic function that corresponds to the given graph.
Start by identifying the vertex of the parabola.
x=0, y=1
Subtract term
(-a)2=a2
LHS+2=RHS+2
LHS/9=RHS/9
ba=b/3a/3
Rearrange equation
A quadratic function is said to be written in factored form, or intercept form, if it follows a specific format.
y=a(x−p)(x−q)
Here, a, p, and q are real numbers with a=0. The value of a gives the direction of the parabola. When a>0, the parabola faces upward, and when a<0, it faces downward. The zeros of the parabola are p and q, and the axis of symmetry is the vertical line x=2p+q.
Consider the graph of y=21(x−7)(x−13).
Direction | Zeros | Axis of Symmetry |
---|---|---|
a=21 | p=7 and q=13 | 2p+q ⇓ 27+13=10 |
Since 21 is greater than 0, the parabola opens upward. | The zeros are 7 and 13. Therefore, the parabola intersects the x-axis at (7,0) and (13,0). | The axis of symmetry is the vertical line x=10. |
Function 1 | Function 2 | Function 3 |
---|---|---|
y=2(x+1)(x−3) ⇕ y=2(x−(-1))(x−3) |
y=(x−5)(x−9) ⇕ y=1(x−5)(x−9) |
y=5x(x−2) ⇕ y=5(x−0)(x−2) |
In the following applet, several quadratic functions are expressed in different forms. Are the quadratic functions written in vertex form, factored form, or neither?
When a quadratic function is written in factored form, some characteristics of its graph can be identified.
y=a(x−p)(x−q) | ||
---|---|---|
Direction | Zeros | Axis of Symmetry |
a>0 a<0 ⇒ upward⇒ downward
|
p and q | x=2p+q |
It is possible to graph a quadratic function using these characteristics. Consider the function y=-(x+1)(x−5).
x=2
Add and subtract terms
Remove parentheses
-a(-b)=a⋅b
By now, it is not a secret that LaShay loves playing golf.
Note that, for the given function, the value of a is -2. Therefore, the parabola opens downward. This corresponds to the loci of the points plotted in the coordinate plane. Finally, these points can be connected with a smooth curve to draw the parabola.
Continuing his studies, Mark wants to write the factored form of the quadratic function that corresponds to the given parabola.
Start by identifying the x-intercepts of the parabola.
x=-2, y=-3
Add and subtract terms
a(-b)=-a⋅b
LHS/(-9)=RHS/(-9)
-b-a=ba
ba=b/3a/3
Rearrange equation
For the following quadratic functions, identify the vertex or the zeros.
(a−b)2=a2−2ab+b2
Commutative Property of Multiplication
Multiply
Calculate power
Distribute -2
Distribute (x−3)
Distribute -2x & 2
Add terms
Consider the following graph of a quadratic function.
Let's begin by recalling the intercept form of a quadratic function. y=a(x-p)(x-q) In this form, p and q are the x-intercepts, or zeros, of the function. Let's take a look at our graph to identify the intercepts.
As we can see, the x-intercepts of the parabola are x=-1 and x=2. Let's use this information to begin to write the given function in intercept form. f(x)=a(x-(-1))(x-2) ⇕ f(x)=a(x+1)(x-2) Now, since the parabola passes through the point (-2,2), we can substitute -2 for x and 2 for f(x) and solve for a.
Finally, we write the completed intercept form of the equation of the function. f(x)=1/2(x+1)(x-2)
Consider the following graph of a quadratic function.
Let's begin by recalling the vertex form of a quadratic function. y=a(x-h)^2+k In this form, ( h, k) is the vertex of the parabola. Now let's take another look at our graph and see if we can find the vertex.
We can see that the vertex of the given function is at ( 1, -1). Let's use this information to start writing the vertex form of the given function. f(x)=a(x- 1)^2+( - 1) ⇕ f(x)=a(x-1)^2-1 Next, since the parabola passes through the point (2,1), we can substitute 2 for x and 1 for f(x) and solve for a.
Finally, we can write the completed vertex form of the function. f(x)=2(x-1)^2-1