x2-1=(x-1)(x+1)
It factors out like this because it's the difference of two squares. It only works with subtraction and is always in the format with two groups of parenthases. For example: 16x-4. 16 and 4 are both squares, so it factors out like this: (4x+2)(4x-2). One set is always minus, the other is always plus. The order doesn't matter.
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