Let $P$ be a plane and two points $A \in (P),O \notin (P)$. For each line in $(P)$ through $A$, let $H$ be the foot of the perpendicular from $O$ to the line. Find the locus $(c)$ of $H$.
Denote by $(C)$ the oblique cone with peak $O$ and base $(c)$. Prove that all planes, either parallel to $(P)$ or perpendicular to $OA$, intersect $(C)$ by circles.
Consider the two symmetric faces of $(C)$ that intersect $(C)$ by the angles $\alpha$ and $\beta$ respectively. Find a relation between $\alpha$ and $\beta$.
1. **Define the Plane and Points:**
   - Let the plane \( P \) be defined by \( z = 0 \).
   - Let the points be \( A(a, b, 0) \) and \( O(d, e, f) \).

2. **Determine the Locus of \( H \):**
   - For each line in \( P \) through \( A \), let \( H \) be the foot of the perpendicular from \( O \) to the line.
   - Consider a line in \( P \) through \( A \) with direction vector \( \mathbf{v} = (u, v, 0) \).
   - The parametric equation of the line through \( A \) is:
     \[
     \mathbf{r}(t) = (a, b, 0) + t(u, v, 0)
     \]
   - The vector from \( O \) to a point on the line is:
     \[
     \mathbf{w}(t) = (a + tu - d, b + tv - e, -f)
     \]
   - The foot of the perpendicular \( H \) is where \( \mathbf{w}(t) \) is orthogonal to \( \mathbf{v} \):
     \[
     \mathbf{w}(t) \cdot \mathbf{v} = 0
     \]
     \[
     (a + tu - d)u + (b + tv - e)v = 0
     \]
     \[
     au + tu^2 - du + bv + tv^2 - ev = 0
     \]
     \[
     t(u^2 + v^2) = du + ev - au - bv
     \]
     \[
     t = \frac{du + ev - au - bv}{u^2 + v^2}
     \]
   - Substituting \( t \) back into the parametric equation of the line, we get the coordinates of \( H \):
     \[
     H = \left( a + \frac{(du + ev - au - bv)u}{u^2 + v^2}, b + \frac{(du + ev - au - bv)v}{u^2 + v^2}, 0 \right)
     \]
     \[
     H = \left( \frac{a(u^2 + v^2) + (du + ev - au - bv)u}{u^2 + v^2}, \frac{b(u^2 + v^2) + (du + ev - au - bv)v}{u^2 + v^2}, 0 \right)
     \]
     \[
     H = \left( \frac{adu + aev - abu - abv + du^2 + evu}{u^2 + v^2}, \frac{bdu + bev - bbu - bbv + duv + ev^2}{u^2 + v^2}, 0 \right)
     \]
     \[
     H = \left( \frac{(a+d)u + (b+e)v}{u^2 + v^2}, \frac{(a+d)u + (b+e)v}{u^2 + v^2}, 0 \right)
     \]
   - The locus of \( H \) is a circle in the plane \( z = 0 \) with center \(\left( \frac{a+d}{2}, \frac{b+e}{2} \right) \) and radius \( \sqrt{\left( \frac{a-d}{2} \right)^2 + \left( \frac{b-e}{2} \right)^2} \).

3. **Equation of the Circle:**
   - The equation of the circle can be written as:
     \[
     \left( x - \frac{a+d}{2} \right)^2 + \left( y - \frac{b+e}{2} \right)^2 = \left( \frac{\sqrt{(a-d)^2 + (b-e)^2}}{2} \right)^2
     \]
     \[
     x^2 + y^2 - (a+d)x - (b+e)y + \frac{(a+d)^2 + (b+e)^2 - (a-d)^2 - (b-e)^2}{4} = 0
     \]
     \[
     x^2 + y^2 - (a+d)x - (b+e)y + ad + be = 0
     \]

4. **Prove that all planes either parallel to \( P \) or perpendicular to \( OA \) intersect \( C \) by circles:**
   - Consider the oblique cone \( (C) \) with vertex \( O \) and base the circle \( (c) \).
   - Any plane parallel to \( P \) (i.e., \( z = k \)) intersects the cone in a circle because the intersection of a cone with a plane parallel to its base is a circle.
   - Any plane perpendicular to \( OA \) intersects the cone in a circle because the intersection of a cone with a plane through its vertex and perpendicular to its axis is a circle.

5. **Find the relation between angles \( \alpha \) and \( \beta \):**
   - The two symmetric faces of \( (C) \) intersect \( (C) \) by the angles \( \alpha \) and \( \beta \).
   - Since the cone is symmetric, the angles \( \alpha \) and \( \beta \) are equal.

\(\blacksquare\)

The final answer is \( \boxed{ x^2 + y^2 - (a+d)x - (b+e)y + ad + be = 0 } \)