Math Made Easy: Tackling Problems with the Discriminant

Welcome to the world of mathematics, where we often encounter complex problems and equations. In this article, we will dive into the realm of the discriminant—a powerful tool that helps us tackle math problems, especially those involving quadratic equations.

Whether you’re a student looking to ace your math exams or someone interested in the magic of mathematics, understanding and using the discriminant can make your math journey much more manageable.

The discriminant is a mathematical tool crucial in solving various problems and equations. It’s particularly valuable when dealing with quadratic equations, offering insights into their nature and solutions.

By grasping the concept of the discriminant and how to apply it, you can simplify complex math problems, gain confidence in your mathematical abilities, and ultimately achieve success in your math endeavours.

What Is the Discriminant?

Let’s start at the beginning. What exactly is the discriminant? The discriminant is a fundamental concept primarily associated with quadratic equations in mathematics. It’s a mathematical expression crucial in determining the nature of these equations’ roots (solutions).

The Formula

The discriminant of a quadratic equation, typically denoted by Δ (Delta), is calculated using the following formula:

The formula $x = \dfrac{-b \pm \sqrt{b^2-4ac}}{2a}$ gives the solutions to the general quadratic equation $ax^2+bx+c=0$. By examining the expression under the square root sign, $b^2-4ac$, we know that the discriminant symbol used is $\Delta$.

The quadratic formula becomes $x=\dfrac{-b + \sqrt{\Delta}}{2a}$ and $x=\dfrac{-b-\sqrt{\Delta}}{2a}$.

Understanding the Discriminant’s Significance

Now that we have the formula, let’s understand why the discriminant is so significant. It’s all about the roots of the quadratic equation. The discriminant helps us determine whether these roots are real, repeated, or complex.

Real and Distinct Roots (Δ > 0)

When Δ is greater than zero, the quadratic equation has two real and distinct roots. In practical terms, this means there are two different solutions to the equation.

$\Delta > 0$
If the discriminant is positive, there are two distinct solutions. We can determine more information than this by checking whether the discriminant is a perfect square.
Case 1:
If $2x^2-7x-4=0$, then $a=2,b=-7$ and $c=-4$.
\( \begin{align} \displaystyle
x &=\dfrac{-b \pm \sqrt{b^2-4ac}}{2a} \\
&= \dfrac{7 \pm \sqrt{81}}{2 \times 2} \\
&= \dfrac{7 \pm 9}{4} \\
&= 4 \text{ or } x=-\dfrac{1}{2} \\
\Delta &= b^2-4ac \\
&= (-7)^2 -4 \times 2 \times (-4) \\
&= 81
\end{align} \)
The quadratic trinomial will have two rational solutions if the discriminant is positive and has a perfect square. This means the quadratic trinomial can be factorised easily; that is $2x^2-7x-4=(2x+1)(x-4)$.
Case 2:
If $x^2-5x-1=0$ then $a=1,b=-5$ and $c=-1$.
\( \begin{align} \displaystyle
x &=\dfrac{-b \pm \sqrt{b^2-4ac}}{2a} \\
&= \dfrac{5 \pm \sqrt{29}}{2 \times 1}\\
&= \dfrac{5 \pm \sqrt{29}}{2}\\
\Delta &= b^2-4ac \\
&= (-5)^2-4 \times 1 \times (-1) \\
&= 29
\end{align} \)
If the discriminant is positive but not a perfect square, this indicates that the factors are irrational. In such cases, the quadratic formula comes into play to find the two irrational (surd) solutions.

Real and Repeated Roots (Δ = 0)

If Δ equals zero, the equation has real but repeated roots. This means there’s only one solution, but it’s repeated.

$\Delta = 0$
If $x^2+8x+16=0$, then $a=1,b=8$ and $c=16$.
\( \begin{align} \displaystyle
x &=\dfrac{-b \pm \sqrt{b^2-4ac}}{2a} \\
&= \dfrac{-8 \pm \sqrt{0}}{2}\\
&= -4 \\
\Delta &= b^2-4ac \\
&= 8^2-4 \times 1 \times 16 \\
&= 0
\end{align} \)
If the discriminant is equal to zero, then the two solutions are the same. This may be regarded as one rational solution equal to $ \displaystyle -\dfrac{b}{2a}$.
This is, if $b^2-4ac=0$, then $\displaystyle x=\dfrac{-b+\sqrt{0}}{2a}$ which is the same as $\displaystyle x=\dfrac{-b-\sqrt{0}}{2a}$.
One solution indicates that the quadratic trinomial is a perfect square that can be factorised easily using the rule of the perfect square: that is $x^2+8x+16=(x+4)^2$.
We also call this repeated or double root.

Complex Roots (Δ < 0)

When Δ is less than zero, the equation has complex roots. Complex roots come in pairs—conjugates of each other—and they involve imaginary numbers.

$\Delta <0$
If $x^2+2x+3=0$, then $a=1,b=2$ and $c=3$.
\( \begin{align} \displaystyle
x &=\dfrac{-b \pm \sqrt{b^2-4ac}}{2a} \\
&= \dfrac{-2 \pm \sqrt{-8}}{2} \\
\Delta &= b^2-4ac \\
&= 2^2-4 \times 1 \times 3 \\
&= -8
\end{align} \)
There are no real solutions if the discriminant is less than zero (negative) because the expression under the square root sign is negative. Finding a real number, the square root of a negative number is impossible.

Solving Quadratic Equations with the Discriminant

Now, let’s get into the practical part—how to use the discriminant to solve quadratic equations. Suppose you have a quadratic equation with \( ax² + bx + c = 0 \) and want to find its roots. Here’s a step-by-step guide:

Step 1: Calculate the Discriminant

First, calculate Δ using the formula Δ = b² – 4ac. This step is crucial because it tells you which case you’re dealing with—real and distinct roots, real and repeated roots, or complex roots.

Step 2: Determine the Nature of the Roots

Based on the value of Δ, you can determine the nature of the roots:

  • If Δ > 0, you have two real and distinct roots.
  • If Δ = 0, you have real and repeated roots.
  • If Δ < 0, you have complex roots.

Step 3: Find the Roots

Now that you know the nature of the roots, you can find them:

  • For real and distinct roots, use the quadratic formula.
  • Use the same formula with Δ = 0 for real and repeated roots.
  • For complex roots, remember that they come in conjugate pairs.

Applications of the Discriminant

You might wonder where this knowledge of the discriminant is applied in real life. The truth is it’s used in various fields, including physics, engineering, finance, and computer science.

Practical Applications

  • Physics: The discriminant helps solve motion-related problems, such as determining when and where an object hits the ground.
  • Engineering: Engineers use it to analyze the stability of structures, such as bridges and buildings.
  • Finance: In finance, the discriminant can help assess risk and make informed decisions when dealing with financial models.

Understanding the discriminant’s applications can open up a world of possibilities in problem-solving.

Common Mistakes and Pitfalls

As with any mathematical concept, there are common errors that students might make when working with the discriminant. Let’s identify a few of these pitfalls and how to avoid them:

  1. Sign Errors: Mistakes in determining the signs of coefficients can lead to incorrect results. Double-check your signs when plugging values into the formula.
  2. Forgetting to Calculate Δ: Sometimes, students fail to calculate the discriminant in the heat of problem-solving. Always start by finding Δ to know which case you’re in.
  3. Not Using the Correct Formula: Ensure you use the right quadratic formula for your case. The formula can vary depending on whether Δ is positive, zero, or negative.

Example 1

Use the discriminant to determine the nature of the roots of $2x^2-2x+5=0$.

\( \begin{align} \displaystyle
\Delta &= b^2-4ac \\
&= (-2)^2-4 \times 2 \times 5 \\
&= -36
\end{align} \)
Since $\Delta \lt 0$, there are no real roots.

Example 2

Use the discriminant to determine the nature of the roots of $3x^2-4x-2=0$.

\( \begin{align} \displaystyle
\Delta &= b^2-4ac \\
&= (-4)^2-4 \times 3 \times (-2) \\
&= 40
\end{align} \)
Since $\Delta \gt 0$, but $40$ is not a square, there are $2$ distinct irrational roots.

Example 3

Use the discriminant to determine the nature of the roots of $9x^2-6x+1=0$.

\( \begin{align} \displaystyle
\Delta &= b^2-4ac \\
&= (-6)^2 -4 \times 9 \times 1 \\
&= 0
\end{align} \)
Since $\Delta = 0$, the equation has a repeated root.

Example 4

Find the values of $k$ for which $x^2-4x+k=0$ has two distinct real roots.

\( \begin{align} \displaystyle
\Delta &\gt 0 \\
b^2-4ac &\gt 0 \\
(-4)^2-4 \times 1 \times k &\gt 0 \\
16-4k &\gt 0\\
-4k &\gt-16 \\
\therefore k &\lt 4
\end{align} \)

Example 5

Find the values of $k$ for which $kx^2+(k+3)x-1=0$ has real roots.

\( \begin{align} \displaystyle
\Delta &\ge 0 &\text{for real roots} \\
b^2-4ac &\ge 0 \\
(k+3)^2-4 \times k \times (-1) &\ge 0 \\
k^2 + 6k + 9 + 4k &\ge 0\\
k^2 + 10k + 9 &\ge 0\\
(k+1)(k+9) &\ge 0 \\
\therefore k &\le -9 \text{ or } \ge -1
\end{align} \)


In conclusion, the discriminant is a valuable tool in mathematics that helps us solve quadratic equations and understand the nature of their roots. By mastering its applications, you can simplify complex math problems and excel in various fields where math is crucial. So, the next time you encounter a quadratic equation, remember the discriminant—it’s your key to unlocking solutions effortlessly.

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