Andy C. answered • 09/09/17

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The rule for absolute value is

|N| = N if N>0,

-N if N<0,

0 if N=0

The equality holds for x=0

If x>0, then -x<0.

So |-x| = -(-x) = x and the equality holds.

If x<0, then -x>0.

|-x|=-x > 0 and the equality does not hold.

So the equality is valid for all x>=0