Mathématiques

What Are the Roots of an Equation?

Roots, solutions and x-intercepts are three names for one set of numbers. Worked on x² − 5x + 6 = 0, whose roots are exactly where its graph meets the axis.

Roots of an equation

The values of the variable that make an equation true; for an equation written as f(x) = 0 they are exactly where the graph of f meets the x-axis.

Aussi appelé: Solutions, Zeros, x-intercepts
Où les élèves le rencontrent: Grade 8 mathematics, as soon as quadratics are solved by factorising, and constantly thereafter at GCSE, IGCSE and on the Digital SAT.

La réponse en bref
The roots of an equation are the values of the variable that make it true. The roots of x² − 5x + 6 = 0 are 2 and 3. When an equation is written as f(x) = 0, its roots, its solutions and the x-intercepts of the graph of y = f(x) are the same numbers.

Un exemple

x² − 5x + 6 = 0 → (x − 2)(x − 3) = 0 → x = 2 or x = 3

Two numbers multiply to give zero only when one of them is zero, so either x − 2 = 0 or x − 3 = 0. That gives 2 and 3, and substituting either back into the original equation returns zero, which is the only real check there is. Now sketch y = x² − 5x + 6 and the same two numbers appear as the points where the curve crosses the x-axis. Nothing new has happened — the graph is a picture of the equation, so it holds the same answers.

Three words for one set of numbers

Textbooks switch between root, solution and zero without warning, and students reasonably assume there is a distinction to learn. There is barely one. Each word belongs to a slightly different object: an equation has roots and solutions, a function has zeros, and a graph has x-intercepts. Point at the same problem from those four directions and the numbers you get are identical.

The habit worth forming is the translation itself. When a question says find the zeros of f(x) = x² − 5x + 6, it is asking you to solve x² − 5x + 6 = 0. When it says state where the graph crosses the x-axis, it is asking the same thing again. Recognising that saves you from learning three methods for one job.

Roots — of an equation, most common in British papers
Solutions — of an equation, the everyday word
Zeros — of a function, standard in American texts and on the SAT
x-intercepts — of a graph, the same numbers read off the picture

Counting them off the curve

Because roots are crossings, you can count them by looking. A parabola that cuts the axis twice has two roots; one that touches and turns has a single repeated root; one that misses entirely has none. A straight line that is not horizontal has exactly one, which is why linear equations never produce a second answer to check.

This works in reverse too, and that is the more useful direction. Given that a curve has roots at 2 and 3, you can write it as (x − 2)(x − 3) multiplied by some constant, without solving anything. Factorised form and root positions are the same information written two ways.

The other meaning of the word

The word root does double duty in mathematics, and the collision causes real confusion. A square root is an operation. A root of an equation is an answer. They are related historically but they are not the same idea, and a question about one is not a question about the other.

The sharpest illustration is the pair that looks identical. The equation x² = 9 has two roots, 3 and −3, because both of them square to 9. But √9 means 3 alone — the square root sign is defined to return the positive value only. That is why the quadratic formula carries a ± in front of the root: the sign has to be put back by hand, because the symbol will not supply it.

Questions fréquentes

Are roots and solutions the same thing?

For a single equation, yes — they are interchangeable. The one place the words drift apart is simultaneous equations, where a solution usually means a pair of values that satisfies both equations at once. Nobody calls that pair a root, so if a question uses the word root, it is asking about one equation.

How many roots does a quadratic equation have?

Two, one, or none, and the discriminant b² − 4ac tells you which before you solve. Two distinct roots when it is positive, one repeated root when it is zero, no real roots when it is negative. A quadratic can never have three, however many ways the question is dressed up.

Can an equation have no roots?

Yes. x² + 1 = 0 has no real roots, because squaring any real number gives something that is zero or positive, and adding one keeps it positive. Its graph sits entirely above the x-axis and never crosses. Beyond school mathematics that equation does have solutions, but they are not real numbers.

Why does my textbook say zeros instead of roots?

It is probably an American book, or SAT material. Zeros of a function is the standard phrasing there, while British specifications lean on roots and solutions. Students preparing for both GCSE and the Digital SAT meet both words in the same week, which is worth flagging early rather than discovering mid-paper.

Dernière mise à jour 21 août 2026