# Exponential Integration Question

1. Jan 27, 2012

### GreenPrint

Why is $e^{\int \frac{dt}{t}}$ = $e^{ln|t|}$ = t as apposed to |t|? I don't understand what happened to the absolute value operator. Thanks for any help.

I understand that $e^{x}$>0. Is this the justification? But I don't understand why you can't have a negative t in $e^{ln|t|}$ because you would take the absolute value of a negative number.

2. Jan 27, 2012

### Staff: Mentor

It should be |t|, as you thought.

It's possible that there is some other context that you're not including, in which t is assumed to be positive. In that case, |t| = t.

3. Jan 28, 2012

### GreenPrint

If I had the differential equation

$\frac{dy}{dt}$ + $\frac{y}{t}$ = 5

Then using integration factors

y = $\frac{5∫e^{\int \frac{dt}{t}}dt}{e^{\int \frac{dt}{t}}}$ = $\frac{5∫e^{ln|t|}dt}{e^{ln|t|}}$ = $\frac{5∫|t|dt}{|t|}$

I'm unsure how to proceed without ignoring the absolute value functions but it appears ignoring them seems to be just fine for whatever reason

4. Jan 29, 2012

### GreenPrint

bump - went onto second page