# Thread: differentiation of a modulus function

1. ## differentiation of a modulus function

I'm a little confused

f(x) = |x|^2 - 4|x|

I know that gives the equations of:
x^2 - 4x and
x^2 + 4x

and you can differentiate them, but which equation corresponds to which part of the graph?

2. Originally Posted by freswood
I'm a little confused

f(x) = |x|^2 - 4|x|

I know that gives the equations of:
x^2 - 4x and
x^2 + 4x

and you can differentiate them, but which equation corresponds to which part of the graph?

When $x\ge 0$:

$|x|^2 - 4|x|=x^2-4x$.

When $x\le 0$:

$|x|^2 - 4|x|=x^2+4x$.

As a reality check you can always try plugging in numbers and seeing what
you get.

RonL

3. Hello, freswood!

Find the derivtive of: $f(x) \:= \:|x|^2 - 4|x|$
This is equivalent to the piecewise function: $f(x)\:=\:\left\{\begin{array}{cc} x^2 - 4x & x \geq 0 \\ x^2 + 4x & x < 0\end{array}$

Differentiate them for both cases.

You can check your results against the graph.

Code:
                      |
*             |             *
|
*            |            *
- - * - - - - - * - - - - - * - -
-4 *       * | *       * 4
* *    |    * *
|

4. Originally Posted by Soroban
Differentiate them for both cases.
differention of |x|=|x|/x
where x is not equal to 0

5. Thanks!

What confused me is that for something like |x^2 - 4x| one applies to y>0 and the other to y < 0.

as in x < 0 U x > 4
and 0<x<4

Can anyone explain the difference between the two?

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