Your homework assignment is to find the focus of the parabola . You say the focus is . Banu says the focus is . Carlos says the focus is . Which one of you is correct?
Guidance
You learned in the Quadratic Functions chapter that parabolas don’t always have their vertex at . In this concept, we will address parabolas where the vertex is , learn how to find the focus, directrix and graph.
Recall from the previous concept that the equation of a parabola is or and the vertex is on the origin. In the Quadratic Functions chapter, we learned that the vertex form of a parabola is . Combining the two, we can find the vertex form for conics.
If the parabola is horizontal, then the equation will be . Notice, that even though the orientation is changed, the and values remain with the and values, respectively.
Finding the focus and directrix are a little more complicated. Use the extended table (from the previous concept) below to help you find these values.
Notice that the way we find the focus and directrix does not change whether is positive or negative.
Example A
Analyze the equation . Find the vertex, axis of symmetry, focus, and directrix. Then determine if the function opens up, down, left or right.
Solution: First, because is squared, we know that the parabola will open to the left or right. We can conclude that the parabola will open to the right because 8 is positive, meaning that is positive. Next, find the vertex. Using the general equation, , the vertex is and the axis of symmetry is . Setting , we have that . Adding to the -value of the vertex, we get the focus, . Subtracting from the -value of the vertex, we get the directrix, .
Example B
Graph the parabola from Example A. Plot the vertex, axis of symmetry, focus, and directrix.
Solution: First, plot all the critical values we found from Example A. Then, determine a set of symmetrical points that are on the parabola to make sure your curve is correct. If , then is either -7 or 9. This means that the points and are both on the parabola.
It is important to note that parabolas with a horizontal orientation are not functions because they do not pass the vertical line test.
Example C
The vertex of a parabola is and the directrix is . Find the equation of the parabola.
Solution: First, let’s determine the orientation of this parabola. Because the directrix is horizontal, we know that the parabola will open up or down (see table/pictures above). We also know that the directrix is above the vertex, making the parabola open down and will be negative (plot this on an plane if you are unsure).
To find , we can use the vertex, and the equation for a horizontal directrix, .
Now, using the general form, , we can find the equation of this parabola.
Intro Problem Revisit This parabola is of the form . From the table earlier in this lesson, we can see that the focus of a parabola of this form is . So now we have to find h , k , and p .
If we compare to , we see that:
1. or
2. or
3.
From these facts we can find .
Therefore, the focus of the parabola is and Carlos is correct.
Guided Practice
1. Find the vertex, focus, axis of symmetry and directrix of .
2. Graph the parabola from #1.
3. Find the equation of the parabola with vertex and focus .
Answers
1. The vertex is and the parabola opens up because is positive and is squared. , making . The focus is or , the axis of symmetry is , and the directrix is or .
2.
3. The vertex is , so and . The focus is , meaning that that parabola will be horizontal. We know this because the -values of the vertex and focus are both -1. Therefore, is added or subtracted to .
we can infer that and
Therefore, the equation is .
Vocabulary
- Standard Form (of a Parabola)
- or where is the vertex.
Practice
Find the vertex, focus, axis of symmetry, and directrix of the parabolas below.
- Graph the parabola from #1.
- Graph the parabola from #2.
- Graph the parabola from #4.
- Graph the parabola from #5.
Find the equation of the parabola given the vertex and either the focus or directrix.
- vertex: , focus:
- vertex: , directrix:
- vertex: , directrix:
- Challenge focus: , directrix:
- Extension Rewrite the equation of the parabola, , in standard form by completing the square. Then, find the vertex. (For a review, see the Completing the Square when concept.)