Maths Olympiad Prep

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Geometry Difficulty 6.6 National olympiad Find the answer

Let \ell be a line and PP be a point in R3\mathbb{R}^3. Let SS be the set of points XX such that the distance from XX to \ell is greater than or equal to two times the distance from XX to PP. If the distance from PP to \ell is d>0d>0, find Volume(S)\text{Volume}(S).

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. **Define the line \ell and point PP:**
Let \ell be the line defined by the points (d,0,z)(-d, 0, z), and let P=(0,0,0)P = (0, 0, 0).

2. **Set up the inequality for the set SS:**
The set SS consists of points X=(x,y,z)X = (x, y, z) such that the distance from XX to \ell is at least twice the distance from XX to PP. Mathematically, this can be written as:
dist(X,)2dist(X,P) \text{dist}(X, \ell) \geq 2 \cdot \text{dist}(X, P)

3. **Calculate the distance from XX to \ell:**
The distance from a point (x,y,z)(x, y, z) to the line \ell (which is parallel to the zz-axis and passes through (d,0,0)(-d, 0, 0)) is given by:
dist(X,)=(x+d)2+y2 \text{dist}(X, \ell) = \sqrt{(x + d)^2 + y^2}

4. **Calculate the distance from XX to PP:**
The distance from a point (x,y,z)(x, y, z) to the point P=(0,0,0)P = (0, 0, 0) is:
dist(X,P)=x2+y2+z2 \text{dist}(X, P) = \sqrt{x^2 + y^2 + z^2}

5. Set up the inequality:
(x+d)2+y22x2+y2+z2 \sqrt{(x + d)^2 + y^2} \geq 2 \sqrt{x^2 + y^2 + z^2}

6. Square both sides to eliminate the square roots:
(x+d)2+y24(x2+y2+z2) (x + d)^2 + y^2 \geq 4(x^2 + y^2 + z^2)

7. Expand and simplify the inequality:
x2+2dx+d2+y24x2+4y2+4z2 x^2 + 2dx + d^2 + y^2 \geq 4x^2 + 4y^2 + 4z^2
d2+2dx3x2+3y2+4z2 d^2 + 2dx \geq 3x^2 + 3y^2 + 4z^2
3x2+3y2+4z22dxd2 3x^2 + 3y^2 + 4z^2 - 2dx \leq d^2

8. **Complete the square for the xx term:**
3(x22dx3)+3y2+4z2d2 3 \left( x^2 - \frac{2dx}{3} \right) + 3y^2 + 4z^2 \leq d^2
3(xd3)2d23+3y2+4z2d2 3 \left( x - \frac{d}{3} \right)^2 - \frac{d^2}{3} + 3y^2 + 4z^2 \leq d^2
3(xd3)2+3y2+4z24d23 3 \left( x - \frac{d}{3} \right)^2 + 3y^2 + 4z^2 \leq \frac{4d^2}{3}

9. Divide through by 3:
(xd3)2+y2+4z234d29 \left( x - \frac{d}{3} \right)^2 + y^2 + \frac{4z^2}{3} \leq \frac{4d^2}{9}

10. Recognize the equation of an ellipsoid:
The inequality represents an ellipsoid centered at (d3,0,0)\left( \frac{d}{3}, 0, 0 \right) with semi-axes 2d3\frac{2d}{3}, 2d3\frac{2d}{3}, and d3\frac{d}{\sqrt{3}}.

11. Calculate the volume of the ellipsoid:
The volume VV of an ellipsoid with semi-axes aa, bb, and cc is given by:
V=43πabc V = \frac{4}{3} \pi abc
Here, a=2d3a = \frac{2d}{3}, b=2d3b = \frac{2d}{3}, and c=d3c = \frac{d}{\sqrt{3}}.

12. **Substitute the values of aa, bb, and cc:**
V=43π(2d3)(2d3)(d3) V = \frac{4}{3} \pi \left( \frac{2d}{3} \right) \left( \frac{2d}{3} \right) \left( \frac{d}{\sqrt{3}} \right)
V=43π4d29d3 V = \frac{4}{3} \pi \cdot \frac{4d^2}{9} \cdot \frac{d}{\sqrt{3}}
V=43π4d393 V = \frac{4}{3} \pi \cdot \frac{4d^3}{9\sqrt{3}}
V=16πd3273 V = \frac{16 \pi d^3}{27\sqrt{3}}

The final answer is 16πd3273\boxed{\frac{16 \pi d^3}{27\sqrt{3}}}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.