Find the following integral (using Substitution method)
`int x^3/sqrt(x^2+1) dx`

Explanation

To solve the integral \( \int \frac{x^3}{\sqrt{x^2 + 1}} \, dx \) using the substitution method, we can follow these steps:

### Step 1: Choose a substitution
Let \( z = \sqrt{x^2 + 1} \). Then, we can express \( x^2 \) in terms of \( z \):
\[ z^2 = x^2 + 1 \implies x^2 = z^2 - 1 \]

...