My Math Forum  

Go Back   My Math Forum > Math Forums > Math

Math General Math Forum - For general math related discussion and news


Thanks Tree3Thanks
  • 1 Post By JeffM1
  • 1 Post By SDK
  • 1 Post By mathman
Reply
 
LinkBack Thread Tools Display Modes
January 24th, 2019, 02:48 AM   #1
Member
 
Joined: Feb 2018
From: Iran

Posts: 52
Thanks: 3

Derivatives


The answer in my book is 5 but if we put 5 instead of x all of them are differentiable
Elize is offline  
 
January 24th, 2019, 03:02 AM   #2
Global Moderator
 
Joined: Dec 2006

Posts: 20,942
Thanks: 2210

$|x^2 - 4|$ isn't differentiable at ±2 (two values of $x$), $\text{sgn}(x^3 + x)$ isn't differentiable at 0, etc.
skipjack is online now  
January 24th, 2019, 03:10 AM   #3
Senior Member
 
Joined: May 2016
From: USA

Posts: 1,310
Thanks: 551

You are misreading the question. It is not asking for the actual values of x where f(x) is not differentiable. It is asking how many such values there are.
Thanks from idontknow
JeffM1 is offline  
January 24th, 2019, 04:59 AM   #4
SDK
Senior Member
 
Joined: Sep 2016
From: USA

Posts: 635
Thanks: 401

Math Focus: Dynamical systems, analytic function theory, numerics
Unrelated comment: This is truly a terrible question. Whoever assigned it isn't paying attention. The only reasonable answer is 3, not 5. It only makes sense to talk about differentiability of a function on its domain. Functions don't "see" anything that isn't in their domain. Both of the following sentences make an equal amount of sense.

1. $f$ is not differentiable at $x = \frac{1}{\sqrt{2}}$.

2. $f$ is not differentiable at $x = $ 1998 Honda Accord.
Thanks from topsquark

Last edited by skipjack; January 24th, 2019 at 11:52 AM.
SDK is offline  
January 24th, 2019, 01:28 PM   #5
Global Moderator
 
Joined: May 2007

Posts: 6,807
Thanks: 717

$x^2-4=0$, 2 values.
$x^3+x=0$, 1 real value.
$2x^2-1=0$, 2 values.
Total 5 (real) values.
Thanks from idontknow
mathman is offline  
January 24th, 2019, 02:06 PM   #6
Senior Member
 
Joined: Dec 2015
From: somewhere

Posts: 606
Thanks: 88

It is enough to know how to find the derivative , then the domain can be found easily .
idontknow is offline  
Reply

  My Math Forum > Math Forums > Math

Tags
derivatives



Thread Tools
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
How many derivatives does it take? John Travolski Calculus 9 September 6th, 2016 08:37 AM
Are partial derivatives the same as implicit derivatives? johngalt47 Calculus 2 November 23rd, 2013 11:44 AM
derivatives ( help ) r-soy Calculus 2 November 28th, 2010 05:52 PM
what is the derivatives r-soy Calculus 1 November 27th, 2010 08:58 PM
derivatives help r-soy Calculus 2 November 27th, 2010 08:26 AM





Copyright © 2019 My Math Forum. All rights reserved.