Question

How do you multiply `(x + y + 3)(x + y - 4)`?

Answer 1

The Expression`=x^2+2xy+y^2-x-y-12`.

Explanation:

Letting `x+y=z`, we find that,

The Expression`=(z+3)(z-4)=z(z-4)+3(z-4)=z^2-4z+3z-12`

`=z^2-z-12`

Surrendering `z=x+y`, we get,

The Expression`=(x+y)^2-x-y-12`

`=x^2+2xy+y^2-x-y-12`.