# Homework Help: Quick question on exponential decay problem

1. Aug 9, 2011

### ozone

I'm attaching the problem as a png. The top half is the question whereas the second half is the solution. I understand everything about the question until the ultimate answer

the final answer is: (r (constant) x0(constant) / k (constant)) * (1 - e^-60t)
as shown.

However I don't understand why my answer differs. I concluded the problem with
(r (constant) x0(constant) / k (constant)) * (-e^-60t)
This doesn't seem to be a basic algebra issue, and It is beyond my comprehension.

Thank you.

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2. Aug 9, 2011

### ArcanaNoir

Did you have the same integral as they did? If so, just looking at the last step where they calculate the definite integral, you must have made an algebra error. If that's not the problem, can you post your work so we can look for where you made a mistake?

Perhaps you forgot the term with the 0 (the lower bound) when computing your definite integral?

3. Aug 10, 2011

### ozone

I guess it could just be algebraic. I didn't quite understand why it was that they were taking 1 minus the e term. Could you explain please?

4. Aug 10, 2011

### daveb

What is the value of the exponential when the lower limit of integration is taken (t = 0)?

5. Aug 11, 2011

### ozone

oh wow now I see.. Thanks for helping me to see the obvious!
(I ignored the lower bound~)