Algebra II : Graphing Parabolas

Example Questions

Example Question #1 : Graphing Parabolas

All of the following are equations of down-facing parabolas EXCEPT:

Explanation:

A parabola that opens downward has the general formula

,

as the negative sign in front of the  term makes flips the parabola about the horizontal axis.

By contrast, a parabola of the form rotates about the vertical axis, not the horizontal axis.

Therefore, is not the equation for a parabola that opens downward.

Example Question #21 : Parabolic Functions

Consider the equation:

The vertex of this parabolic function would be located at:

Explanation:

For any parabola, the general equation is

, and the x-coordinate of its vertex is given by

.

For the given problem, the x-coordinate is

.

To find the y-coordinate, plug  into the original equation:

Therefore the vertex is at .

Example Question #22 : Parabolic Functions

In which direction does graph of the parabola described by the above equation open?

right

left

up

down

right

Explanation:

Parabolas can either be in the form

for vertical parabolas or in the form

for horizontal parabolas. Since the equation that the problem gives us has a y-squared term, but not an x-squared term, we know this is a horizontal parabola. The rules for a horizontal parabola are as follows:

• If , then the horizontal parabola opens to the right.
• If , then the horizontal parabola opens to the left.

In this case, the coefficient in front of the y-squared term is going to be positive, once we isolate x. That makes this a horizontal parabola that opens to the right.

Example Question #23 : Parabolic Functions

Find the vertex form of the following quadratic equation:

Explanation:

Factor 2 as GCF from the first two terms giving us:

Now we complete the square by adding 4 to the expression inside the parenthesis and subtracting 8 ( because ) resulting in the following equation:

which is equal to

Hence the vertex is located at

Example Question #24 : Parabolic Functions

Based on the figure below, which line depicts a quadratic function?

Green line

Blue line

Red line

None of them

Purple line

Red line

Explanation:

A parabola is one example of a quadratic function, regardless of whether it points upwards or downwards.

The red line represents a quadratic function and will have a formula similar to .

The blue line represents a linear function and will have a formula similar to .

The green line represents an exponential function and will have a formula similar to .

The purple line represents an absolute value function and will have a formula similar to .

Example Question #2 : Graphing Parabolas

Which of the following parabolas is downward facing?

Explanation:

We can determine if a parabola is upward or downward facing by looking at the coefficient of the  term. It will be downward facing if and only if this coefficient is negative. Be careful about the answer choice . Recall that this means that the entire value inside the parentheses will be squared. And, a negative times a negative yields a positive. Thus, this is equivalent to . Therefore, our answer has to be

Example Question #26 : Parabolic Functions

What is the vertex of the function ? Is it a maximum or minimum?

; maximum

; minimum

; maximum

; minimum

; minimum

Explanation:

The equation of a parabola can be written in vertex form: .

The point  in this format is the vertex. If  is a postive number the vertex is a minimum, and if  is a negative number the vertex is a maximum.

In this example, . The positive value means the vertex is a minimum.

Example Question #27 : Parabolic Functions

How many -intercepts does the graph of the function

have?

Four

None of these

One

Two

Zero

Zero

Explanation:

The graph of a quadratic function  has an -intercept at any point  at which , so, first, set the quadratic expression equal to 0:

The number of -intercepts of the graph is equal to the number of real zeroes of the above equation, which can be determined by evaluating the discriminant of the equation, . Set , and evaluate:

The discriminant is negative, so the equation has two solutions, neither of which are real. Consequently, the graph of the function  has no -intercepts.

Example Question #7 : Graphing Polynomials

Which of the following graphs matches the function ?