Circles $ C_1,C_2$ intersect at $ P$. A line $ \Delta$ is drawn arbitrarily from $ P$ and intersects with $ C_1,C_2$ at $ B,C$. What is locus of $ A$ such that the median of $ AM$ of triangle $ ABC$ has fixed length $ k$.
1. **Given Setup**: We have two circles \( C_1(O_1) \) and \( C_2(O_2) \) intersecting at points \( P \) and \( Q \). A line \( \Delta \) through \( P \) intersects \( C_1 \) at \( B \) and \( C_2 \) at \( C \). Let \( M \) be the midpoint of \( BC \) and \( O \) be the midpoint of \( O_1O_2 \).

2. **Construct Perpendiculars**: Construct points \( B' \in C_1 \) and \( C' \in C_2 \) such that \( B'B \perp BC \) and \( C'C \perp BC \). This implies that \( O_1 \) and \( O_2 \) are the midpoints of \( PB' \) and \( PC' \) respectively.

3. **Collinearity and Trapezoid**: It is easy to prove that points \( B', Q, C' \) are collinear. The quadrilateral \( BB'C'C \) forms a right trapezoid because \( BB' \parallel CC' \) and \( \angle B'BC = \angle C'CB = 90^\circ \).

4. **Midpoints and Perpendiculars**: Let \( N \) be the midpoint of \( B'C' \). Since \( O \) is the midpoint of \( PN \) and \( NM \perp BC \) (because \( MN \) is the central median of the right trapezoid \( BB'C'C \)), we have \( OM = OP \).

5. **Conclusion**: Since \( OM = OP \), the point \( M \) moves on the circle with center \( O \) and radius \( OP \).

\[
\boxed{OM = OP}
\]