In a circle we enscribe a regular $2001$-gon and inside it a regular $667$-gon with shared vertices.

Prove that the surface in the $2001$-gon but not in the $667$-gon is of the form $k.sin^3\left(\frac{\pi}{2001}\right).cos^3\left(\frac{\pi}{2001}\right)$ with $k$ a positive integer. Find $k$.
1. **Area of a Regular Polygon**: The area \( A \) of a regular \( n \)-gon inscribed in a circle of radius \( r \) is given by:
   \[
   A = \frac{1}{2} n r^2 \sin\left(\frac{2\pi}{n}\right)
   \]
   This formula can be derived by summing the areas of \( n \) isosceles triangles, each with a central angle of \( \frac{2\pi}{n} \).

2. **Area of the 2001-gon**: Using the formula, the area of the regular 2001-gon is:
   \[
   A_{2001} = \frac{1}{2} \cdot 2001 \cdot r^2 \cdot \sin\left(\frac{2\pi}{2001}\right)
   \]

3. **Area of the 667-gon**: Similarly, the area of the regular 667-gon is:
   \[
   A_{667} = \frac{1}{2} \cdot 667 \cdot r^2 \cdot \sin\left(\frac{2\pi}{667}\right)
   \]

4. **Difference in Areas**: The area of the surface in the 2001-gon but not in the 667-gon is:
   \[
   A_{\text{diff}} = A_{2001} - A_{667} = \frac{1}{2} r^2 \left( 2001 \sin\left(\frac{2\pi}{2001}\right) - 667 \sin\left(\frac{2\pi}{667}\right) \right)
   \]

5. **Simplifying the Expression**: Notice that \( \frac{2\pi}{667} = 3 \cdot \frac{2\pi}{2001} \). Let \( \theta = \frac{\pi}{2001} \). Then:
   \[
   \sin\left(\frac{2\pi}{667}\right) = \sin(6\theta)
   \]
   and
   \[
   \sin\left(\frac{2\pi}{2001}\right) = \sin(2\theta)
   \]

6. **Using Trigonometric Identities**: We use the identity for \( \sin(6\theta) \):
   \[
   \sin(6\theta) = 3 \sin(2\theta) - 4 \sin^3(2\theta)
   \]
   Substituting this into the difference of areas:
   \[
   A_{\text{diff}} = \frac{1}{2} r^2 \left( 2001 \sin(2\theta) - 667 (3 \sin(2\theta) - 4 \sin^3(2\theta)) \right)
   \]
   Simplifying further:
   \[
   A_{\text{diff}} = \frac{1}{2} r^2 \left( 2001 \sin(2\theta) - 2001 \sin(2\theta) + 2668 \sin^3(2\theta) \right)
   \]
   \[
   A_{\text{diff}} = \frac{1}{2} r^2 \cdot 2668 \sin^3(2\theta)
   \]

7. **Expressing in Terms of \( \sin \theta \) and \( \cos \theta \)**: Using the identity \( \sin(2\theta) = 2 \sin(\theta) \cos(\theta) \):
   \[
   \sin^3(2\theta) = (2 \sin(\theta) \cos(\theta))^3 = 8 \sin^3(\theta) \cos^3(\theta)
   \]
   Therefore:
   \[
   A_{\text{diff}} = \frac{1}{2} r^2 \cdot 2668 \cdot 8 \sin^3(\theta) \cos^3(\theta)
   \]
   \[
   A_{\text{diff}} = 10672 r^2 \sin^3(\theta) \cos^3(\theta)
   \]

8. **Conclusion**: The surface area in the 2001-gon but not in the 667-gon is of the form \( k \sin^3\left(\frac{\pi}{2001}\right) \cos^3\left(\frac{\pi}{2001}\right) \) with \( k = 10672 \).

The final answer is \( \boxed{ k = 10672 } \)