Let triangle $ ABC$ acutangle, with $ m \angle ACB\leq\ m \angle ABC$. $ M$ the midpoint of side $ BC$ and $ P$ a point over the side $ MC$. Let $ C_{1}$ the circunference with center $ C$. Let $ C_{2}$ the circunference with center $ B$. $ P$ is a point of $ C_{1}$ and $ C_{2}$. Let $ X$ a point on the opposite semiplane than $ B$ respecting with the straight line $ AP$; Let $ Y$ the intersection of side $ XB$ with $ C_{2}$ and $ Z$ the intersection of side $ XC$ with $ C_{1}$. Let $ m\angle PAX \equal{} \alpha$ and $ m\angle ABC \equal{} \beta$. Find the geometric place of $ X$ if it satisfies the following conditions:
$ (a) \frac {XY}{XZ} \equal{} \frac {XC \plus{} CP}{XB \plus{} BP}$
$ (b) \cos(\alpha) \equal{} AB\cdot \frac {\sin(\beta )}{AP}$
To solve this problem, we need to find the locus of point \( X \) that satisfies the given conditions. Let's break down the problem step by step.

1. **Understanding the Geometry:**
   - \( \triangle ABC \) is an acute triangle with \( \angle ACB \leq \angle ABC \).
   - \( M \) is the midpoint of \( BC \).
   - \( P \) is a point on segment \( MC \).
   - \( C_1 \) is the circle centered at \( C \) with radius \( CP \).
   - \( C_2 \) is the circle centered at \( B \) with radius \( BP \).
   - \( X \) is a point in the opposite semiplane to \( B \) with respect to line \( AP \).
   - \( Y \) is the intersection of \( XB \) with \( C_2 \).
   - \( Z \) is the intersection of \( XC \) with \( C_1 \).
   - \( \angle PAX = \alpha \) and \( \angle ABC = \beta \).

2. **Condition (a):**
   \[
   \frac{XY}{XZ} = \frac{XC + CP}{XB + BP}
   \]
   Since \( Y \) and \( Z \) lie on circles \( C_2 \) and \( C_1 \) respectively, we have:
   \[
   XY = BP \quad \text{and} \quad XZ = CP
   \]
   Therefore, the condition simplifies to:
   \[
   \frac{BP}{CP} = \frac{XC + CP}{XB + BP}
   \]
   This implies:
   \[
   BP \cdot (XB + BP) = CP \cdot (XC + CP)
   \]
   Simplifying, we get:
   \[
   BP \cdot XB + BP^2 = CP \cdot XC + CP^2
   \]
   Since \( BP = CP \) (as \( P \) lies on both circles), the equation simplifies to:
   \[
   BP \cdot XB = CP \cdot XC
   \]
   Thus:
   \[
   XB = XC
   \]
   This implies that \( X \) must lie on the perpendicular bisector of \( BC \).

3. **Condition (b):**
   \[
   \cos(\alpha) = AB \cdot \frac{\sin(\beta)}{AP}
   \]
   This condition relates the angle \( \alpha \) with the sides of the triangle and the position of \( P \). Since \( \alpha = \angle PAX \), we need to find \( X \) such that this trigonometric condition holds.

4. **Combining Both Conditions:**
   From condition (a), we know that \( X \) lies on the perpendicular bisector of \( BC \). For condition (b), we need to ensure that \( \cos(\alpha) = AB \cdot \frac{\sin(\beta)}{AP} \).

   Since \( \alpha = \angle PAX \), we can use the Law of Cosines in \( \triangle APX \) to express \( \cos(\alpha) \) in terms of the sides of the triangle. However, since \( X \) lies on the perpendicular bisector of \( BC \), it is equidistant from \( B \) and \( C \).

   Therefore, the locus of \( X \) is the perpendicular bisector of \( BC \).

The final answer is the perpendicular bisector of \( \boxed{ BC } \).