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Solving a Quadratic Equation Using a Graph

(KS4, Year 10)

Let's start with a test!

The Test

Here is a -question, multi-choice test for the "Solving a Quadratic Equation Using a Graph" lesson. The pass mark is 90%. Don't worry! All the information you need to pass is in the lesson section under the test.

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The Lesson

A quadratic equation can be solved by drawing the equation on a graph and seeing where the curve crosses the x-axis. Imagine you wanted to solve the quadratic equation x² − 3x + 2. Plot y = x² − 3x + 2 on a graph and read off where the curve crosses the x-axis. solve_quadratic_equation_graph

We can see that x = 1 and x = 2 solve the quadratic equation.

How to Solve Quadratic Equations Using a Graph

Solving a quadratic equation using a graph is easy.

Question

Solve the quadratic equation shown below using a graph.

Step-by-Step:

1

Draw the quadratic equation on a pair of axes. solve quadratic equation graph step 1

2

Find the x-coordinate of the first point where the curve crosses the x-axis. The curve crosses the x-axis at x = −1, which is a solution to the quadratic equation.

3

Find the x-coordinate of the second point where the curve crosses the x-axis. The curve crosses the x-axis at x = 3, which is also a solution to the quadratic equation.

Answer:

We have solved the quadratic equation: x = −1, x = 3.

Lesson Slides

Sometimes quadratic equations have repeated roots: the same value of x solves the quadratic equation twice. The slider below shows another real example of how to solve a quadratic equation using a graph.

3 Cases of Roots on a Graph

There are 3 possible cases for the roots of a quadratic equation.

2 real, distinct roots. Occurs when the curve crosses the x-axis in two places.
1 repeated root. Occurs when the curve touches the x-axis at one point.
2 complex roots. Occurs when the curve does not touch the x-axis at all.