How can I differentiate modulus?

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Discussion Overview

The discussion revolves around the differentiation of the modulus function, specifically the function y=|x+4|. Participants explore the correct approach to finding the derivative and the implications of the modulus on the function's behavior, including points of minimum and undefined derivatives.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions their differentiation process, suggesting that their method leads to an incorrect minimum point.
  • Another participant agrees that the derivative cannot equal zero and notes that it is undefined at x=-4.
  • A different participant challenges the initial steps, stating that y^2 does not equal x+4.
  • Following the correction, it is noted that the derivative remains undefined at x=-4, leading to a form of 0/0.
  • Another approach is proposed, where the absolute value is removed by defining the function piecewise, leading to constant derivatives in each piece and highlighting the undefined derivative at x=-4.
  • It is mentioned that extreme points occur where the derivative is zero, undefined, or at the endpoints of the domain.

Areas of Agreement / Disagreement

Participants generally agree that the derivative cannot equal zero and is undefined at x=-4. However, there is disagreement regarding the initial differentiation steps and the validity of the proposed methods.

Contextual Notes

There are limitations in the initial differentiation approach, particularly regarding the incorrect assumption that y^2 equals x+4, which affects the subsequent calculations. The discussion also highlights the need to consider the piecewise nature of the modulus function.

Who May Find This Useful

This discussion may be useful for students or individuals studying calculus, particularly those interested in differentiation techniques involving absolute value functions.

ojsimon
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Ok so i was wondering if what i am doing is correct, But it gets the wrong minimum point?
So my function is y=|x+4|

1) y^2=x+4
2)2y(dy/dx)=1
3)dy/dx = 1/2y
4)dy/dx = 1/2|x+4|

I set that 0 and get
0=1/(2(|x+4|))

Am i write in thinking this cannot be solved? or missing something?

Thanks
 
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Yep that's right. It's the same as asking what number x can be used to make 1/x=0? None of course. And at x=-4 the derivative is undefined.
 
It goes wrong from the start, y^2 \neq x+4.
 
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Oh right, I guess I brushed over it too fast.

When that mistake is fixed, you'll still find the the derivative cannot equal zero anywhere and it's still undefined at x=-4 with the form 0/0
 
Oh yeah, thanks,
 
Another approach is to get rid of the absolute values by writing the function as
y = x + 4, x >= -4
y = -(x + 4), x < -4

Then y' = 1 for x > -4 and y' = -1 for x < - 4. y' does not exist at x = -4.

Any extreme points of a function occur at places where y' = 0, or y' is undefined, or at finite endpoints of the domain in cases where a function is defined only on an interval [a, b].
 

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