I was hoping to get a bit of feedback on a proof I've done involving absolute values. This problem is taken from D. Velleman's How to Prove it (#3.5.11). I've only written on one side of the biconditional, but the I believe the other conditional can be structured similarly.
Problem Statement:
Prove that for every real number $x$, $\lvert 2x – 6 \rvert \gt x $ iff $\lvert x – 4 \rvert \gt 2 $.
My question:
Do the statements below constitute a valid proof or do they require supplemental explanation?
($\rightarrow$) suppose $ 2x – 6 \gt 0$. Per the definition of $ \lvert 2x – 6 \rvert $ we proceed by cases.
Case 1: $ 2x – 6 \geq 0 $.
Since $ 2x – 6 \geq 0 $, $ \lvert 2x – 6 \rvert = 2x – 6.$
Then
$$ \lvert 2x – 6 \rvert \gt x $$
$$= 2x – 6 \gt x $$
$$= -6 \gt -x $$
$$= 6 \lt x $$
$$= 4+2 \lt x $$
$$= 2 \lt x-4 $$
$$= 2 \lt \lvert x – 4 \rvert $$
$$= \lvert x-4 \rvert \gt 2 $$
Case 2: $ 2x – 6 \lt 0 $. Since $ 2x – 6 \lt 0 $, $ \lvert 2x – 6 \rvert = 6 – 2x $.
Then
$$ \lvert 2x – 6 \rvert \gt x $$
$$= 6 – 2x \gt x $$
$$= 6 \gt 3x $$
$$= 2 \gt x $$
$$= 4 > x + 2 $$
$$= 4 – x \gt 2 $$
$$= \lvert x – 4 \rvert \gt 2 $$
EDIT
I removed the word 'equivalence' from the problem because it might not be the most appropriate word to use. I'm mostly just asking whether the series of statements I've given provide a valid proof of the conclusion.
Best Answer
Looks pretty good to me. But there is an important point you should change:
Concerning the proof line by line (if you want to dot every "i"):