Some expression require a certain amount of recogintion before they can be factorised.

The expressioncan be factorised, but only when it is recognised that the first two terms can be factorised as a difference of squares:

The last two terms can also be factorised asso thatis a common factor.

Some expressions with four terms, involving four different variables, can also be factorised.

The simplest is

Notice thatis a common factor of the first two terms, andis a common factor of the last two terms. We have

Nowis a common factor, so we can write

Hence