The Discriminant: How Many Real Roots?

Ontario Grade 11 Mathematics

Quadratic Functions

How one number predicts where a quadratic graph meets the x-axis

A quadratic equation and its graph describe the same relationship in different ways. Solving the equation tells you which input values make the output zero. On the graph, those same input values mark where the curve meets the x-axis. The discriminant gives a quick way to predict how many such values there are, even before finding them. This lesson connects these three ideas: roots, x-intercepts, and the discriminant.

What you will learn

Explain how a root of a quadratic equation connects to an x-intercept of its graph.
Calculate the discriminant of a quadratic equation.
Use the discriminant to tell whether a quadratic has two, one, or no real roots and x-intercepts.

1. Prerequisite bridge: equations and graphs

A quadratic expression has a squared variable as its highest power. A quadratic function can be written as

f(x)=ax^2+bx+c

, where

a

,

b

, and

c

are numbers and

a\ne 0

. Its graph is a curve called a parabola.

A root is an

x

-value that makes an equation true. For a quadratic function, roots are found by setting the output to zero:

ax^2+bx+c=0

. The phrase real root means a solution that is a real number, such as an integer, fraction, or decimal.

An x-intercept is a point where a graph meets or touches the horizontal x-axis. Every point on that axis has a vertical coordinate of zero. So, to find the x-intercepts of

y=f(x)

, set

y=0

and solve

f(x)=0

.

f(x)=ax^2+bx+c,\quad a\ne 0

Roots are input values that make the quadratic output zero.
An x-intercept is a point on the graph with vertical coordinate zero.
The root value becomes the x-coordinate of an x-intercept.

2. Connecting roots and x-intercepts

Consider the equation

x^2-4=0

. It has the solutions

x=-2

and

x=2

, because each value makes the left side equal zero. For the function

y=x^2-4

, the corresponding graph points are

(-2,0)

and

(2,0)

. These are its x-intercepts.

This connection works for every quadratic function. To move from an equation to the graph, solve the equation after setting the output to zero. To move from the graph to the equation, read the x-coordinates where the curve meets the x-axis. The x-coordinate is the root; the full point includes the zero vertical coordinate.

A graph can meet the x-axis twice, touch it once, or miss it. These three visual possibilities match the number of real roots: two, one, or none. A curve that touches the axis at one point has one distinct root, even though that root is repeated when the equation is factored.

f(r)=0\iff (r,0) is an x-intercept

A root $r$ corresponds to the x-intercept $(r,0)$ .
The number of distinct real roots matches the number of x-intercepts.
A repeated root gives one x-intercept, not two different points.

3. The discriminant predicts the number of roots

For a quadratic equation in the form

ax^2+bx+c=0

, the discriminant is the expression

b^2-4ac

. It is calculated from the three coefficients. The discriminant does not give the roots by itself; it tells how many real roots to expect.

A positive discriminant means there are two different real roots. The graph therefore has two x-intercepts. A zero discriminant means there is one repeated real root and one x-intercept where the graph touches the axis. A negative discriminant means there are no real roots and no x-intercepts.

The sign matters, not just the size. For example, a discriminant of

16

is positive, so there are two real roots. A discriminant of

-16

is negative, so there are no real roots. When calculating, use the coefficients with their signs. In

x^2+5x-6=0

, for instance,

c

is

-6

, not

6

.

This rule applies when the equation is written as

ax^2+bx+c=0

. If needed, first rearrange the equation so one side is zero, then identify

a

,

b

, and

c

. Keep any negative signs attached to their coefficients.

\Delta=b^2-4ac

Positive discriminant: two real roots and two x-intercepts.
Zero discriminant: one repeated real root and one x-intercept.
Negative discriminant: no real roots and no x-intercepts.

4. Reading the result in context

The discriminant links an algebraic calculation to a graph feature. It lets you predict how many times a parabola meets the x-axis without first finding the exact root values. If a question asks for the actual intercepts, the discriminant alone is not enough; you must also solve the quadratic equation.

In an application, a root can represent an input that makes a measured quantity zero. For example, if a function describes an object's height relative to the ground, a real root may represent a time when the height is zero. The discriminant can indicate whether there are two such input values, one, or none, within the real-number model. The meaning of a root still depends on the situation, so check whether its value makes sense in context.

Use the three representations together: the equation shows the roots, the graph shows the x-intercepts, and the discriminant predicts their number. Each representation answers a slightly different question, but all describe the same quadratic relationship.

Use the discriminant to predict the number of real x-intercepts.
Solve the equation when exact root values are required.
Check whether a root is meaningful for the situation being modelled.

Discriminant, roots, and graph connection

Discriminant	Number of real roots	Number of x-intercepts	What the graph does
$\Delta>0$	Two different roots	Two	Crosses the x-axis twice
$\Delta=0$	One repeated root	One	Touches the x-axis once
$\Delta<0$	No real roots	None	Does not meet the x-axis

Worked example

Find and connect the roots and intercepts

For

f(x)=2x^2+x-3

, find the discriminant, determine the number of real roots, and identify the x-intercepts.

Identify the coefficients
The function is already in the form $ax^2+bx+c$ . Read each coefficient, keeping its sign. This gives the values needed for the discriminant.
$a=2,\quad b=1,\quad c=-3$
Calculate the discriminant
Substitute the coefficients into $b^2-4ac$ . Because $c$ is negative, the product $4ac$ is negative, and subtracting it increases the result.
$\Delta=1^2-4(2)(-3)=25$
Predict the number of roots
The discriminant is positive, so the equation has two different real roots. Its graph has two x-intercepts.
$\Delta>0$
Find the intercept values
Set the function equal to zero and factor. The two factors show which values make the product zero. Each root gives an x-intercept by pairing it with a vertical coordinate of zero.
$(2x+3)(x-1)=0;\quad x=-\frac{3}{2},\ 1$

Answer: The discriminant is

25

. There are two real roots,

x=-\frac{3}{2}

and

x=1

. The x-intercepts are

\left(-\frac{3}{2},0\right)

and

(1,0)

.

Check: Substituting either root into a factor makes that factor zero. The two distinct roots agree with the positive discriminant.

Common mistakes and how to avoid them

Treating an x-intercept as only an x-value.

Correction: The root is the x-value. The x-intercept is the point formed by that value and zero, such as

(r,0)

.

Using the wrong sign for a coefficient when calculating the discriminant.

Correction: Read

a

,

b

, and

c

from the equation after setting one side to zero. Keep each coefficient's sign.

Saying a zero discriminant gives two different x-intercepts.

Correction: A zero discriminant gives one repeated root, so the graph touches the x-axis at one point.

Assuming a negative discriminant gives an x-intercept with a negative x-coordinate.

Correction: A negative discriminant means there are no real roots, so the graph has no x-intercepts. It does not describe the sign of an x-coordinate.

Lesson summary

Set a quadratic function equal to zero to find its roots.
Each real root $r$ corresponds to the x-intercept $(r,0)$ .
Calculate the discriminant using $\Delta=b^2-4ac$ .
A positive, zero, or negative discriminant means two, one, or no real x-intercepts, respectively.

Check your understanding

Question 1

A quadratic equation has discriminant

0

. How many distinct real roots and x-intercepts does it have?

Two roots and two x-intercepts
One repeated root and one x-intercept
No real roots and no x-intercepts
correctIndex

Show answer and explanation

One repeated root and one x-intercept

A zero discriminant means there is one repeated real root. That root gives one point where the graph touches the x-axis.

Question 2

For

3x^2-2x+4=0

, what does the discriminant tell you?

There are two real roots and two x-intercepts.
There is one repeated real root and one x-intercept.
There are no real roots and no x-intercepts.
correctIndex

Show answer and explanation

There are no real roots and no x-intercepts.

Here

\Delta=(-2)^2-4(3)(4)=4-48=-44

. Since it is negative, there are no real roots or x-intercepts.

Question 3

A quadratic graph has one x-intercept at

(4,0)

. What is the corresponding root?

$x=0$
$x=4$
$y=4$
correctIndex

Show answer and explanation

x=4

The x-coordinate of the intercept is the root, so the root is

x=4

.

Key terms

Quadratic function: A function that can be written as $f(x)=ax^2+bx+c$ , where $a\ne 0$ .
Root: An input value that makes an equation or function output equal to zero.
X-intercept: A point where a graph meets the x-axis; its vertical coordinate is zero.
Discriminant: The expression $b^2-4ac$ for a quadratic equation written as $ax^2+bx+c=0$ .
Repeated root: A root that occurs twice in the factors of a quadratic but gives just one distinct x-value.

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A1.7 · The Discriminant: How Many Real Roots?

Ontario Grade 11 Mathematics

How one number predicts where a quadratic graph meets the x-axis

What you will learn

1. Prerequisite bridge: equations and graphs

2. Connecting roots and x-intercepts

3. The discriminant predicts the number of roots

4. Reading the result in context

Discriminant, roots, and graph connection

Find and connect the roots and intercepts

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through MCF3M

Tutor support for A1.7 · Connect roots, x-intercepts, and the discriminant

About this lesson and its review