What are roots and the discriminant?

The discriminant of a quadratic equation will tell you how many roots the quadratic equation has.

Solutions of a quadratic equation are known as roots; they can be seen on the graph of a quadratic where the graph crosses the x-axis. Recall that the general quadratic equation: ax^2+bx+c=0, can have two distinct roots, one repeated root or no roots at all (unless you are working with complex numbers).

Here are three examples that illustrate how quadratics can have different numbers of roots:

discriminant

2 distinct roots

discriminant

1 repeated root

discriminant

no roots

Note that a negative quadratic was used to illustrate that a quadratic may have no roots but it is also possible for a positive quadratic to have no roots as well.


The value of the discriminant

The discriminant can tell you how many roots a quadratic equation will have without having to actually find them.

For the quadratic equation ax^2+bx+c=0, the discriminant is given by b^2-4ac.

  • Firstly, if b^2-4ac \hspace{2pt}\textgreater\hspace{2pt} 0, the equation has two distinct roots.
  • Alternatively, if b^2-4ac = 0, the equation has one repeated root.
  • Lastly, if b^2-4ac\hspace{2pt}\textless\hspace{2pt}0, the equation has no roots.

Example 1

Given that the quadratic equation kx^2-4x+2 has equal roots, find the value of k.

Example 2

A quadratic has equation y=px^2+3px-5.

  1. Find an expression, in terms of p, for the discriminant of the quadratic.
  2. Given that the quadratic has two distinct roots, find the possible values of p.


Click here for the definition of a discriminant for higher order polynomials.


Alternatively, click here to find Questions by Topic and scroll down to all past DISCRIMINANT questions to practice some more.


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