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What does the discriminant look like?
It looks like .. a number
5,2, 0 1  this is the discriminant for 4 different equations
What is the discriminant anyway?
The discriminant is a number that can be calculated from any quadratic equation
A quadratic equation is an equation that can be written as $$ ax^2 +bx + c $$ ( where $$a \ne 0 $$)
What is the formula for the Discriminant?
The discriminant in a quadratic equation is found by the following formula and the discriminant provides critical information regarding the nature of the roots/solutions of any quadratic equation.
discriminant= b² − 4ac
Example of the discriminant Quadratic equation = y = 3x² + 9x + 5
 The discriminant = 9 ² − 4 • 3 •5
Important pre requisites
To understand what the discriminant does, it's important that you have a good understanding of
What does the graph of a quadratic equation look like:
What is the solution of a quadratic equation:
The solution can be thought of in two different ways.
Algebraically, the solution occurs when y = 0. So the solution is where $$y =\color{Magenta}ax^2 + \red bx + \color{Blue}{c} $$ becomes $$0 =\color{Magenta}ax^2 + \red bx + \color{Blue}{c} $$
Graphically, since y = 0 is the xaxis, the solution is where the parabola intercepts the xaxis. (This only works for real solutions).
In the picture below, the left parabola has 2 real solutions (red dots), the middle parabola has 1 real solution (red dot) and the right most parabola has no real solutions (yes, it does have imaginary ones)
What does this formula tell us?
The discriminant tells us the following information about a quadratic equation:
 If the solution is a real number or an imaginary number.
 If the solution is rational or if it is irrational.
 If the solution is 1 unique number or two different numbers
Nature of the Solutions
Value of the discriminant  Type and number of Solutions  Example of graph 
Positive Discriminant
b² − 4ac > 0 
Two Real SolutionsIf the discriminant is a perfect square the roots are rational. Otherwise, they are irrational.  
Positive and a Perfect Square
b² − 4ac = perfect square 
Two Real Rational Solutions.  
Positive and a not a perfect square
b² − 4ac = not a perfect square 
Two Real Irrational Solutions.  
Discriminant is Zero
b² − 4ac = 0 
One Real Solution  
Negative Discriminant
b² − 4ac < 0 
No Real Solutions Two Imaginary Solutions 
Example
Quadratic Equation: y = x² + 2x + 1
 a = 1
 b = 2
 c = 1
The discriminant for this equation is
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{2^2}  4\color{Magenta}{(1)}\color{Blue}{(1)} = 0 $$
Since the discriminant is zero, there should be 1 real solution to this equation.
Below is a picture representing the graph and one solution of this quadratic equation Graph of y = x² + 2x + 1
Practice Problems
In this quadratic equation,y = 1x² − 2x + 1
 a = 1
 b = − 2
 c = 1
Using our general formula, the discriminant is
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(2)^2}  4\color{Magenta}{(1)}\color{Blue}{(1)} = 4 $$
Since the discriminant is zero, we should expect 1 real solution which you can see pictured in the graph below.
In this quadratic equation, y = x² − x − 2 and its solution
 a = 1
 b = − 1
 c = − 2
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(1)^2}  4\color{Magenta}{(1)}\color{Blue}{(2)} \\ 1  8 = 1 + 8 = 9 $$
Since the discriminant is positive and rational , there should be 2 real rational solutions to this equation. As you can see below, if you use the quadratic formula to find the actual solutions, you do indeed get 2 real rational solutions.
In this quadratic equation, y = 1x² − 1
 a = 1
 b = 0
 c = − 1
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(0)^2}  4\color{Magenta}{(1)}\color{Blue}{(1)} = 4 $$
Since the discriminant is positive and a perfect square, we have two real solutions that are rational.
Again if you'd like to see the actual solutions and the graph, just look below:
In this quadratic equation, y = x² + 4x − 5
 a = 1
 b = 4
 c = − 5
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(4)^2}  4\color{Magenta}{(1)}\color{Blue}{(5)} \\ 16  4(5) = 16 +20 \\ = 36 $$
Since this quadratic equation's discriminant is positive and a perfect square, there are two real solutions that are rational.
In this quadratic equation, y = x²  4x + 5
 a = 1
 b = 4
 c = 5
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(4)^2}  4\color{Magenta}{(1)}\color{Blue}{(5)} \\ = 16  20 = 4 $$
Since the discriminant is negative , there are no real solutions to this quadratic equation. The only solutions are imaginary.
Below is a picture of this quadratic's graph
y = x² + 4
 a = 1
 b = 0
 c = 4
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(0)^2}  4\color{Magenta}{(1)}\color{Blue}{(4)} = 16 $$
Since the discriminant is negative , there are two imaginary solutions to this quadratic equation.
The solutions are 2i and 2i
Below is a a picture of this equations graph
y = x² + 25
 a = 1
 b = 0
 c = 25
$$ \color{Red}{b^2}  4\color{Magenta}{a}\color{Blue}{c} \\ \color{Red}{(0)^2}  4\color{Magenta}{(1)}\color{Blue}{(25)} = 100 $$
Since the discriminant is negative , there are two imaginary solutions to this quadratic equation.
The solutions are 5i and 5i

This Page:
 Discriminant Formula
 Nature of the Roots