I need to find the Integral with respect to x of (e^(-.02x))/(8-e^(-.02x))

I use u=8-e^(-.02x)

and find that dx= du/.02e^(-.02x) or dx=50du/e^(-.02x)

when i substitute that back into the integral, the integral reduces to:

integral of (50/u)du which equals 50ln(abs(u))+ c

after substituting back in for u I finished with

50ln(abs(8-e^(-.02x)))

Where did I mess up?