# Math Help - Expand

1. ## Expand

Hi

What do i get when i expand:

x^2 - 1

Is it
(x-1)(x+1)

2. Originally Posted by taurus
Hi

What do i get when i expand:

x^2 - 1
expand? there is nothing to expand here, you'd get just that. did you mean factorize? this is the difference of two squares, $x^2 - 1 = (x + 1)(x - 1)$

3. yes thanks

4. Just a quick question:
is it ok to write

21 - 4x - x^2

as

x^2 + 4x - 21?

5. Originally Posted by taurus
Just a quick question:
is it ok to write

21 - 4x - x^2

as

x^2 + 4x - 21?
not unless it is equated to something where you can distribute the minus on the other side as well:

so, say you had: 21 - 4x - x^2 = 0

then it is okay to multiply both sides by -1 to get: x^2 + 4x - 21 = 0

but if you just have the expression by itself, you cannot do that, the best you can do is to say:

$21 - 4x - x^2 = - \left( x^2 + 4x - 21 \right)$

leave the minus outside of the brackets

6. so for example this:

1/(21-4x-x&2)

Which one do i use?

7. Originally Posted by taurus
so for example this:

1/(21-4x-x&2)

Which one do i use?
it is not equated to anything.

$\frac 1{21 - 4x - x^2} = \frac {-1}{x^2 + 4x - 21}$