Integral of e^(-ax) from evaluated between neg/pos infinity

[kkowalko]

Hey guys I am working on a Physics problem and can't figure out a little but of math involved. What is this integral? I tried it out and got "1/a + a", assuming "1/a" is the positive half of the integral.

 

[Gigasoft]

The integral is infinite.

 

[zachzach]

Graph $$f(x) = e^{-x}$$ and see if you can tell what the area under it is.