20.6 Factorising using common factors from groups of terms
By the end of this section you should:
● be able to create equivalent expressions by factorising expressions using common factors
● recognise when to factorise using a common factor to all terms or to group terms fi rst
Type 2: Highest common factor – Groups of terms Consider the expression 2xy + 2xc + 6mn + 4mq We cannot divide all terms in this expression by a common factor, but each pair of terms has a HCF. We look at pairs (or groups) of terms in which we can divide by a common factor. In this case, we can divide 2xy + 2xc by 2x (1st group) and we can divide 6mn + 4mq by 2m (2nd group)
Factorising by grouping within an expression Group terms that have a common factor together in pairs (rearrange order if necessary)
1
2 3
Factorise each pair of terms separately (using the HCF method as before) such that the two 2nd brackets are identical.
Place the common factor (2nd brackets) in a new 1st set of brackets and the uncommon terms (1st brackets) in a new 2nd set of brackets.
Worked example 1 Factorise fully the expression xy − 5y + 2x − 10
Case 1: If the pairs of terms do not have a common factor, other than 1, you may have to rearrange the order of the terms.
For example, the expression ab + 2 + a + 2b needs to be rearranged before we can factorise by grouping. We can try pairing the terms without 2s, and pairing the two other terms with 2s: ab + a+ 2b + 2 Now a is the HCF of the 1st pair and 2 is the HCF of the 2nd pair: