Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math - Questions and Answers from our Archives
_____________________________________________
Associated Topics || Dr. Math Home || Search Dr. Math
_____________________________________________

Calculate the Derivative for Sqrt(x^5)

Date: 03/09/2003 at 14:24:17
From: Rob
Subject: Derivatives

How do I calculate the derivative for the square root of (x to the 
fifth)?

I can do the derivative of (sqrt of x) to the fifth, but not the 
other way around. 

I can get the answer using the calculator: (5*x^4)/2*sqr rt(x^5). 
I know the expression sqrt(x^5) is the same as (x^5)^(1/2), which 
should be x^(5/2), which would mean the derivative is (5/2)x^(3/2), 
but this is not right.


Date: 03/10/2003 at 10:56:26
From: Doctor Ian
Subject: Re: Derivatives

Hi Rob,

You're correct that

  (sqrt(x))^5 = (x^(1/2))^5

              = x^(5/2)

And in fact, the derivative _is_ (5/2)x^(3/2):

  d/dx x^(5/2) = (5/2)x(5/2 - 1)

               = (5/2)x^(3/2)

To see that this is the same as the answer you got from the 
calculator, we can set them equal to each other, and simplify:

                       5x^4
     (5/2)x^(3/2) = -----------         
                    2 sqrt(x^5)


                      5x^4
         5x^(3/2) = -----------        
                     sqrt(x^5)


                       x^4
          x^(3/2) = -----------        
                     sqrt(x^5)


                       x^4
          x^(3/2) = -----------        
                      x^(5/2)


   x^(3/2)x^(5/2) = x^4


          x^(8/2) = x^4


              x^4 = x^4

This statement is true, so the expressions we started with are equal. 

So how did the calculator come up with such a complicated answer? 
Instead of simplifying, it applied the chain rule.  Letting 

  u(x) = x^5, 

this gives us 

  d/dx  sqrt(u) = (1/2)u^(-1/2) du/dx

                = (1/2)u^(-1/2) 5x^4

                    5x^4
                = ---------
                  2 sqrt(u)

                    5x^4
                = ------------
                  2 sqrt(x^5)

This, by the way, is a good illustration of why many teachers feel
that students are becoming too dependent on calculators. Your
approach was much more elegant than the calculator's, and obviously
correct, but when the calculator gave you an answer that looked
different than yours, you assumed that yours must be wrong.  

The truth is, calculators are very fast, and very accurate, but
they're not all that bright:

   Are Calculators Smart?
   http://mathforum.org/library/drmath/view/57030.html 

I hope this helps. Write back if you'd like to talk more about this,
or anything else. 

- Doctor Ian, The Math Forum
  http://mathforum.org/dr.math/ 
Associated Topics:
High School Calculators, Computers
High School Calculus
High School Exponents

Search the Dr. Math Library:


Find items containing (put spaces between keywords):
 
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. MathTM
© 1994-2013 The Math Forum
http://mathforum.org/dr.math/