Evaluate: `int1/(e^x+1)\ dx`

Explanation

To evaluate the integral \( \int \frac{1}{e^x + 1} \, dx \), we can follow these steps:

### Step 1: Substitution
Let \( t = e^x + 1 \). Then, the derivative of \( t \) with respect to \( x \) is:
\[ dt = e^x \, dx \quad \Rightarrow \quad dx = \frac{dt}{e^x} = \frac{dt}{t - 1} \]
This substitution transforms the integral.
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