Evaluate: `int1/(sqrt(1+x^2))dx`

Explanation

To evaluate the integral \( I = \int \frac{1}{\sqrt{1+x^2}} \, dx \), we will use the substitution method. Here are the steps:

### Step 1: Substitution
Let \( x = \tan \theta \). Then, the differential \( dx \) is given by:
\[ dx = \sec^2 \theta \, d\theta \]

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