# Exp. integral

1. Aug 21, 2006

### fredgarvin22

hello

could someone give me a pointer here.

this integral
∫ln(x + c)dx

my guess is, by integration by parts
(ab)' = a'b + ab'
∫ba = ab - ∫b'a

so here
a = ln(c + x) b = c + x
a' = 1/(c + x) b' = 1

ab = (c + x)*ln(c + x)
and
∫b'a = ∫ ((c + x)/(hc + x)) dx
= ∫dx = x
so ab - ∫b'a = (c + x)*ln(c + x) - x

would this be correct?

2. Aug 21, 2006

### quasar987

I think you messed up a little bit in your re-writting the problem, but the answer (c + x)*ln(c + x) - x is correct. When it comes to integrals, you can always verify your answer by differentiating your answer. If it gives the integrand, you've got the right answer. If not, there's a mistake.