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This topic is part of the TCS FREE high school mathematics 'How-to Library', and will help you to how to factorise quadratic expressions.    (See the index page for a list of all available topics in the library.) To make best use of this topic, you need to download the Algematics software. Click here for instructions.

Theory:

A quadratic expression is one where the highest power on the variable is 2. Here are some examples of quadratic expressions:

1.         x², 

2.         z² + 3z, 

3.         2x² + 3x - 5,

4.         a² - 1, 

5.         p² - 4p - 6

When these quadratic expressions are factorised, they are written as the product of two factors.

This is how to factorise these quadratic expressions:

Example 1:      x²

Write as a product: x´x

Example 2:      z² + 3z

Take z out as a common factor:  z(z + 3)

Example 3:      2x² + 3x - 5

This expression can be factorised if the middle term: 3x,
        is replaced by the equivalent expression: -2x+5x,

like this:

                        2x² - 2x + 5x - 5

 After doing this, the first two terms and the last two terms can be factorised:

2x(x - 1) + 5(x - 1)

Now there are only two terms, and there is a common factor: (x - 1), so the expression can be factorised again, like this:

(x - 1)(2x + 5)

Example 4:      a² - 1

This expression can be expressed as: a² - 1² that is a difference of perfect squares, a special pattern that you need to be able to recognise. It can be written immediately as:

(a + 1)(a - 1)

Example 5:      p² - 4p - 6

In this case, the middle term cannot be split to allow factorising as in example 3. You use a technique called 'completing the square' to write the quadratic as a difference of perfect squares, as in example 4 above.

As a difference of perfect squares, the expression becomes:

then it factorises like this:

and simplifies to give:

The ‘method’ section below describes how to use Algematics to factorise examples 3, 4 and 5 above.

Method:

IMPORTANT: This topic assumes that you know how to enter mathematical formulas into Algematics. Find out how by completing the three simple tutorials in the 'Getting Started' section of the Algematics program 'Help'.

There are several methods for factorising quadratic expressions, and the Algematics ‘factorise’ and ‘complete the square’ commands can will guide you through the solutions step by step.

Step 1  Enter the data

Click  and type the expression to be factorised into the maths box in the data entry dialog box.

If the ‘EMPTY’ message is not displayed between the blue buttons, click the  button until the message: ‘INSERT’ appears. If necessary, use the ‘ * ’ symbol for multiply, and the ‘ / ’ symbol for divide.

       Maths...

and then click

Step 2  Factorise

Click  (factorise) once. Several things can happen:

1.    If the terms have common factors, like example 2, these will be taken outside the brackets.

2.    If the expression is a difference of perfect squares, like example 4, then it will be factorised immediately.

3.    If you entered a quadratic expression with three terms, like example 3, then Algematics will attempt to split the middle term. If it is successful, you must click  (factorise) twice more to complete the factorisation.

NOTE: To find out how to split the middle terms of quadratic expressions yourself, click here.

4.   If you entered a quadratic expression with three terms and  (factorise) does not change the expression, then you can try completing the square as described in the box below: