A circle with center O inscribed in a convex quadrilateral ABCD is tangent to the lines
AB, BC, CD, DA at points K, L, M, N respectively. Assume that the lines KL and MN
are not parallel and intersect at the point S. Prove that BD is perpendicular OS.

I think it is very good and beautiful problem. I solved it without help. I'm wondering is it a well known theorem? Also I'm interested who is the creator of this problem? I'll be glad to see simple solution of this problem.
1. **Lemma**: Let \( A \) and \( B \) be two arbitrary points (different from the center of the circle \( O \)). Then \( A \) is on the polar of \( B \) if and only if \( B \) is on the polar of \( A \).

2. **Proof of Lemma**: 
   - Consider the circle with center \( O \) and let \( P \) be a point on the circle. The polar of \( P \) is the line perpendicular to the radius \( OP \) at \( P \).
   - If \( A \) is on the polar of \( B \), then the line \( AB \) is perpendicular to the line joining \( B \) to the point where the polar of \( B \) intersects the circle.
   - By the definition of polar, if \( A \) is on the polar of \( B \), then \( B \) must be on the polar of \( A \).

3. **Application to the Problem**:
   - Given that \( S \) lies on the polar of \( B \) and \( D \), it follows that \( B \) and \( D \) lie on the polar of \( S \).
   - The polar of \( S \) is perpendicular to the line \( OS \) (by the property of polars with respect to a circle).

4. **Conclusion**:
   - Since \( B \) and \( D \) lie on the polar of \( S \), and the polar of \( S \) is perpendicular to \( OS \), it follows that the line \( BD \) is perpendicular to \( OS \).

\[
\boxed{\text{BD is perpendicular to OS}}
\]