Which equation does NOT have the set of all numbers as its solution?

1. |-x| = |x|

2. ||x|| = |x|

3. |-x| = -(-(-x))

4. -(-x) = x

I need help with this problem.

2 answers

3.

Becouse :

| - x | = x

- ( - ( - x ) ) = - x

| - x | = - ( - ( - x ) ) mean:

x = - x

That is true only for x = 0
Actually, since -(-(-x)) = -x
we have
|-x| = -x

which is true for all x <= 0

Still the correct answer, though.