Posted by W on Monday, November 21, 2011 at 12:12am.
How do I integrate (e^(3/x))/(x^2) from 1 to 3. I understand how and why I need to do usubstitution, with u=3/x and so dx=(3 du)/x^2. After plugging (3 du)/x^2 in for dx, I get a really messy problem that I don't know how to finish. Can you please explain the step?
Thanks for your help!

Calculus  Steve, Monday, November 21, 2011 at 4:46am
I think you may be going about things backwards. If
u = 3/x
du = 3/x^2 dx
and your integrand becomes e^u du/(3) = 1/3 Int(e^u du) = 1/3 e^u = 1/3 e^3/x
check: taking the derivative of 1/3 e^3/x we get
1/3 * e^3/x * 3/x^2 = e^(3/x) / x^2
So, evaluating at 3 and 1,
1/3 e^(3/3) + 1/3 e^(3/1) = 1/3 (e^3  e)
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