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Where would you start if you were told to plot:

$$||x-1|-1|$$

I looked at just $f(x) = |x-1|$ and noticed that the two equations are: $\pm (x-1)$ for $x \geq 1$ and $x < 1$. Extrapolating then: $\pm(\pm(x-1)-1)$, but how would I know which regions which equation would work for?

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  • $\begingroup$ Carefully think about it, for example if $x\geq 2$, there is no need for the absolute value, hence $||x-1|-1|=x-2$ for $x\geq 2$. $\endgroup$
    – Mathematician 42
    Apr 29, 2016 at 18:50

3 Answers 3

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1) $y=|x|$

enter image description here

2) $y=|x-1|$ enter image description here

3) $y=|x-1|-1$

enter image description here

4) $y=||x-1|-1|$

enter image description here

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    $\begingroup$ plus 1, because it is always nice to see visually $\endgroup$
    – Naz
    Apr 29, 2016 at 19:23
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    $\begingroup$ +1; Also, there's a vertical bar missing in the formula for the fourth picture. $\endgroup$
    – Daan Michiels
    Apr 29, 2016 at 20:03
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If you know the graph of $f(x)$, then the graph of $|f(x)|$ is just the result of “folding” $f$ about to the $x$ axis. Hence, plotting $f(x) = \bigl\lvert \lvert x - 1 \rvert - 1 \bigr\rvert$ goes like this.

  1. Plot $y = x - 1$ and fold the portion of it that lies below the $x$ axis over the $x$ axis. The result is the graph of $\lvert x - 1 \rvert$.

  2. Then shift this graph downward by $1$ and do the same folding again. Then you will get $\bigl\lvert \lvert x - 1 \rvert - 1 \bigr\rvert$.

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    $\begingroup$ It's more like folding it across the $x$ axis, not reflecting it. Reflecting it would be negation; the positive parts would become negative as well as the negative parts becoming positive. $\endgroup$
    – user2357112
    Apr 29, 2016 at 19:42
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    $\begingroup$ Please listen to user2357112 and don't use the word "reflection" because "reflection" has a specific meaning in math which is different from what you want it to mean in this answer. Somebody at the level of the OP could become terribly confused. $\endgroup$
    – Fixed Point
    Apr 29, 2016 at 23:08
  • $\begingroup$ @FixedPoint fixed. $\endgroup$
    – dezdichado
    Apr 30, 2016 at 4:16
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    $\begingroup$ You still have the word "reflection" in the very first line. $\endgroup$
    – Fixed Point
    Apr 30, 2016 at 6:45
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Start by drawing $y = |x|$. Now move it right one and you get the graph of $y = |x - 1|$. Next yank it down one and get the graph of $y = |x-1|-1$. Finally, reflect up the stuff below the $x$-axis and you are done.

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