• Support PF! Buy your school textbooks, materials and every day products Here!

Problem with improper integrals

  • Thread starter cesaruelas
  • Start date
  • #1
53
0

Homework Statement



∫xe^(-2x)dx from x = 0 to ∞

Homework Equations



-xe^(-2x)/2 - e^(-2x)/4 + C

The Attempt at a Solution



lim b→∞ -x/2e^(2b) - 1/4e^(2b) = 0

wolfram alpha says its 1/4 and I do not know why (it does not show steps)

Can you help me?
 

Answers and Replies

  • #2
ehild
Homework Helper
15,427
1,824
You did not take the lower limit into account. Substitute x=0 and subtract.

ehild
 
  • #3
53
0
You're right lol I'm dumb... anyway substituting the lower limit I get 3/4 and not 1/4 :/
-(-1/2 - 1/4) = 3/4
 
  • #4
53
0
Thank you
 
  • #5
53
0
just saw my mistake... 1/2 should be zero, that way I do get 1/4... thank you again
 

Related Threads on Problem with improper integrals

  • Last Post
Replies
2
Views
756
  • Last Post
Replies
2
Views
782
Replies
8
Views
1K
Replies
4
Views
971
  • Last Post
Replies
1
Views
546
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
11
Views
1K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
5
Views
2K
Replies
3
Views
985
Top