# Precalculus : Graph a Quadratic Function Using Intercepts, Vertex, and Axis of Symmetry

## Example Questions

### Example Question #1 : Graph A Quadratic Function Using Intercepts, Vertex, And Axis Of Symmetry

Find the vertex, roots, and the  value that the line of symmetry falls on of the function .

vertex , the roots  and , and the axis of symmetry would fall on .

vertex , the roots  and , and the axis of symmetry would fall on x=.5.

vertex , the roots  and , and the axis of symmetry would fall on .

vertex , the roots  and , and the axis of symmetry would fall on .

vertex , the roots  and , and the axis of symmetry would fall on .

vertex , the roots  and , and the axis of symmetry would fall on .

Explanation:

All quadratic functions have a vertex and many cross the x axis at points called zeros or roots. If we know the vertex and its zeros, quadratic functions become very easy to draw since the vertex is also a line of symmetry (the zeros are equidistant from the vertex on either side).

Factor the equation  to get  and . Thus, the roots are 3 and -2.

The vertex can be found by using .

simplify

.

The axis of symmetry is halfway between the two roots, or simply the x coordinate of the vertex. So the axis of symmetry lies on x=1/2. To graph, draw a point at the coordinate pair of the vertex. Then draw points on the x axis at the roots, and finally, trace upwards from the vertex through the roots with a gentle curve.

### Example Question #2 : Graph A Quadratic Function Using Intercepts, Vertex, And Axis Of Symmetry

Which of the following functions matches the provided parabolic graph?

Explanation:

Finding the vertex, intercept and axis of symmetry are crucial to finding the function that corresponds to the graph:

The vertex form of a quadratic function is written as:

and the coordinates for the vertex are:

Looking at the graph and the position of the axis of symmetry, the vertex is positioned at , leaving us with an equation so far of:

While we don't know a right away,  is the only option that really works. The y-intercept is at  and we can plug that into the formula to confirm that this is the correct function:

### Example Question #3 : Graph A Quadratic Function Using Intercepts, Vertex, And Axis Of Symmetry

Which of the following is an equation for the parabola represented in the graph below?

Explanation:

Immediately we can tell that the equation has a negative coefficient, because the parabola opens downward, forming an umbrella shape. Based upon the information given in the figure, we can use the intercepts, axis of symmetry, and the vertex to identify the equation of the parabola. Let's observe the vertex form of a parabola written as the following:

In this equation,  is the vertex of the parabola, and  determines whether the parabola opens upwards or downwards. The axis of symmetry is at  and the vertex is located at , which we can plug into the following function:

We know that  is negative because of the position of the parabola.

### Example Question #4 : Graph A Quadratic Function Using Intercepts, Vertex, And Axis Of Symmetry

Where does the vertex of the quadratic  lie?

Explanation:

To find the vertex of a quadratic equation, you'll look to put the quadratic in the form , where  is then the vertex.  To get from the original equation to vertex form, you'll have to complete the square by looking at the terms that include  and  to turn that into a perfect square.  Here you should see that with  as the first two terms, you could have a perfect square if you could use .  So to complete the square, you can express the given quadratic as:

Note that the +1 and -10 terms net out to the -9 that was in the original equation, so in this case you have not altered the value at all, but have merely reallocated numbers to fit vertex form (also note that there was no coefficient to the  term, making the  term in vertex form equal to 1).

From here you can factor the quadratic on the left to perfectly math vertex form:

This means that  and , making the vertex .

### Example Question #5 : Graph A Quadratic Function Using Intercepts, Vertex, And Axis Of Symmetry

What are the vertex and the line of symmetry for the quadratic ?

Vertex: (1, 3)

Line of Symmetry: x = 1

Vertex: (1, -5)

Line of Symmetry: x = 1

Vertex: (5, -1)

Line of Symmetry: x = 5

Vertex: (-3, -1)

Line of Symmetry: x = -3

Vertex: (-1, 5)

Line of Symmetry: x = 5

Vertex: (1, -5)

Line of Symmetry: x = 1

Explanation:

Note that solving for the x-coordinate of the vertex of a parabola also tells you its line of symmetry, so your job here is to put the quadratic into Vertex Form in order to find the vertex, which will give you the line of symmetry. Vertex form is , where  is then the vertex. To get to that form, you will want to complete the square by looking at the  and  terms and determining which perfect square equation they belong to. To do that, separate those two terms from the -2 term, and then factor out the coefficient of 3:

Then note that the way to turn  into a perfect square would be to add 1 to it to get to . Of course, you cannot just add one within the parentheses without balancing the rest of the equation on both sides.  Since that +1 will be multiplied by a coefficient of 3, you should add 3 to the right side of the equation to match what you've done on the left:

Then you can factor the quadratic on the left into perfect square form, and subtract 3 from both sides to reset to 0:

This provides you with Vertex Form, so you can say that  and , making the vertex  and the Line of Symmetry just the x-coordinate of .

### Example Question #6 : Graph A Quadratic Function Using Intercepts, Vertex, And Axis Of Symmetry

What is the line of symmetry of the parabola formed by ?

x=2

x=4

x=-4

x=1

x=-1