Problem with evaluation of integration



Hi all,
I guess this is a professional group working with Maxima, so my inquiry for
basic knowledge of Maxima may not be very attractive for you to explore. But
I will really appreciate your help.
My question is:
When I'm using the integrate function to evaluate an expression, sometimes
Maxima gives me a value after calculation. But sometimes it just gives me a
formal mathematical expression. For example, when I enter this:
integrate(e^(-x),x,1,5);
I get an expression with the integration symbol, the limits and everything,
instead of the evaluation result.
Can you please tell me how to get the numbers I'm looking for?
Thanks,
Chi Ben