What am I doing wrong here?

  • #1

Main Question or Discussion Point

Hey guys

I've been reading through a few books and I can't seem to work this out;
[tex]\int _0^{\infty }e^{-a x^2}xdx = \frac{1}{2a}[/tex]
I keep getting
[tex]\int _0^{\infty }e^{-a x^2}xdx = -\frac{1}{2a}[/tex]
I do the old variable switch
[tex]u = x^2[/tex]
[tex]du=2 x dx[/tex]
Which leaves me with
[tex]\int_0^{\infty } \frac{e^{-a u}}{2} \, du[/tex]
Which them leads me to
[tex]\text{Lim} u\to 0\frac{e^{- a u}}{-2 a}=-\frac{1}{2a}[/tex]
Where am I picking up this unwanted - from?
I can't see where I've gone wrong
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
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hey genericusrnme! :smile:
Where am I picking up this unwanted - from?
0 is the lower limit … so there's an extra minus :wink:
 
  • #3
man I feel stupid now :L

thanks tiny tim :D
 

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