Question:Factorise ax + ay -x -y .
Answer ax + ay -x -y=a(x+y) - (x+y) =(x+y) (a-1). Notice that when the last two terms are grouped with a minus sign outside. the signs of both terms in the bracket must be changed. ax + ay -x -y=a(x+y) - (x+y) and not a(x+y) - (x-y) . It is good plan to remove the brackets mentally to check your working.
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Factorise ax + ay -x -y .