My Math Forum  

Go Back   My Math Forum > College Math Forum > Calculus

Calculus Calculus Math Forum


Thanks Tree1Thanks
  • 1 Post By skeeter
Reply
 
LinkBack Thread Tools Display Modes
February 16th, 2015, 09:07 AM   #1
Senior Member
 
Joined: Sep 2013
From: Earth

Posts: 827
Thanks: 36

Checking mistake

Where did I make mistake ?
My answer is $\displaystyle \frac{1}{4}\pi^2+2$. The correct answer is $\displaystyle \frac{1}{4}\pi^2-2$.
Attached Images
File Type: jpg image (2).jpg (83.3 KB, 4 views)

Last edited by jiasyuen; February 16th, 2015 at 09:14 AM.
jiasyuen is offline  
 
February 16th, 2015, 09:19 AM   #2
Senior Member
 
MarkFL's Avatar
 
Joined: Jul 2010
From: St. Augustine, FL., U.S.A.'s oldest city

Posts: 12,211
Thanks: 522

Math Focus: Calculus/ODEs
The second time you applied integration by parts, you did not distribute the negative sign to the resulting integral.
MarkFL is offline  
February 16th, 2015, 09:31 AM   #3
Math Team
 
skeeter's Avatar
 
Joined: Jul 2011
From: Texas

Posts: 3,101
Thanks: 1677

obviously a sign error ...

$\displaystyle \int x^2\cos{x} \, dx$

$\displaystyle u = x^2$ ; $\displaystyle du = 2x \, dx$

$\displaystyle dv = \cos{x} \, dx$ ; $\displaystyle v = \sin{x}$

$\displaystyle \int x^2\cos{x} \, dx = x^2\sin{x} - \int 2x\sin{x} \, dx$

$\displaystyle u = 2x$ ; $\displaystyle du = 2 \, dx$

$\displaystyle dv = \sin{x}$ ; $\displaystyle v = -\cos{x} \, dx$

$\displaystyle \int x^2\cos{x} \, dx = x^2\sin{x} - \left[-2x\cos{x} - \int -2\cos{x} \, dx\right]$ ... from this step to the next is where I think you made a sign error

$\displaystyle \int x^2\cos{x} \, dx = x^2\sin{x} + 2x\cos{x} - \int 2\cos{x} \, dx$
Thanks from MarkFL
skeeter is offline  
February 16th, 2015, 03:07 PM   #4
Senior Member
 
Joined: Sep 2013
From: Earth

Posts: 827
Thanks: 36

Thanks a lot guys. I wonder when will the time I be one of you guys.
jiasyuen is offline  
February 16th, 2015, 04:03 PM   #5
Senior Member
 
MarkFL's Avatar
 
Joined: Jul 2010
From: St. Augustine, FL., U.S.A.'s oldest city

Posts: 12,211
Thanks: 522

Math Focus: Calculus/ODEs
Quote:
Originally Posted by jiasyuen View Post
Thanks a lot guys. I wonder when will the time I be one of you guys.
You are like me...that's exactly the kind of mistake I could make, and then just not see it.
MarkFL is offline  
February 16th, 2015, 06:56 PM   #6
Global Moderator
 
CRGreathouse's Avatar
 
Joined: Nov 2006
From: UTC -5

Posts: 16,046
Thanks: 938

Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms
As Jesus once said, let he who has not made a sign error throw the first stone. Something like that, anyway.
CRGreathouse is offline  
Reply

  My Math Forum > College Math Forum > Calculus

Tags
checking, mistake



Thread Tools
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Checking mistake. SXmath Calculus 1 February 9th, 2015 05:52 AM
Checking Mistake. jiasyuen Calculus 3 January 30th, 2015 06:16 PM
Can someone fix my mistake livestrong136 Calculus 1 May 8th, 2012 05:02 AM
Is there any mistake bobar77 Algebra 8 February 17th, 2012 10:37 PM
where is the mistake?? islam Calculus 6 December 3rd, 2010 08:33 PM





Copyright © 2019 My Math Forum. All rights reserved.