Derivative of Absolute Value Confusion

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Discussion Overview

The discussion revolves around the derivative of the absolute value function, specifically comparing different expressions for the derivative and addressing potential confusion arising from a calculus homework problem. The scope includes mathematical reasoning and technical clarification related to derivatives.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant notes a discrepancy between their derived expression for the derivative of the absolute value function, x/|x|, and the expression given in a textbook, |x|/x, questioning whether these are equal.
  • Another participant confirms the equality of the two expressions but points out that both are undefined at x=0, suggesting that issues may arise when applying these derivatives near this point.
  • A participant presents a specific function, y=|x|+1/x, and their derived expression for its derivative, which includes x/|x|, contrasting it with a solution manual's expression that appears different, raising concerns about its correctness.
  • There is speculation that the discrepancy in the solution manual could be a typo or miscommunication regarding the derivative expression.
  • One participant clarifies that the confusion was due to an algebra comprehension issue rather than a misunderstanding of the derivative itself.
  • A final participant discusses the implications of the absolute value function's piecewise nature, indicating that the derivative can take different values depending on the sign of x.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the derivative expressions and whether they are equal, indicating that the discussion remains unresolved regarding the specific expressions and their application.

Contextual Notes

There is an acknowledgment that the derivative of the absolute value function is not defined at x=0, which may lead to complications in problems involving this point. Additionally, there are concerns about potential typographical errors in the solution manual.

Who May Find This Useful

This discussion may be useful for students and educators involved in calculus, particularly those grappling with the concept of derivatives of piecewise functions and the absolute value function.

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Quick question (hopefully): I'm helping my girlfriend with some calculus homework over the phone and it looks like the book has a different answer for the derivative of an absolute value than I'm seeing in other places. I'm seeing it as (and I derived it myself to be) x/abs(x) whereas the book (apparently) has abs(x)/x. But it seems to me that these should be equal. Are they?

Why this matters is if you try to apply this to problems, you get some very ugly things.

What am I missing?
 
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x2=|x|2
x*x = |x|*|x|
x/|x| = |x|/x

So yes these things are equal. The ugly things might be coming up because the function is not differentiable at 0 and of course neither of these is defined at zero. If your problem involves crossing x=0 weird things can happen. Can you give an example?
 
Given: y=|x|+1/x
Solution: y=(x2)1/2+x-1
y'=1/2(x2)-1/22x-x-2
y'=x/|x|-1/x2
Can also be written: y'= (x|x|-1)/x2

But unless I'm mishearing her, the solutions manual says:
y'=|x|/(x-1/x2)
...and they don't look equal to me.
 
Maybe it's just a typo (or a mishear), intended to be y'=|x|/x - 1/x2.
 
Yeah, I asked several times, but it is possible that it is either.
 
Yeah, I got her on a webcam and it was an algebra comprehension issue. That's what I thought but I wanted to make sure I wasn't crazy.
 
y = |x|
implies y = ax and y = -ax
therefore y' = a and y' = -a
 

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