GeometryDifficulty 6.6National olympiadFind the answer
Let ℓ be a line and P be a point in R3. Let S be the set of points X such that the distance from X to ℓ is greater than or equal to two times the distance from X to P. If the distance from P to ℓ is d>0, find Volume(S).
A number or a short expression. Spacing and $ signs are ignored.
Solution
1. **Define the line ℓ and point P:** Let ℓ be the line defined by the points (−d,0,z), and let P=(0,0,0).
2. **Set up the inequality for the set S:** The set S consists of points X=(x,y,z) such that the distance from X to ℓ is at least twice the distance from X to P. Mathematically, this can be written as: dist(X,ℓ)≥2⋅dist(X,P)
3. **Calculate the distance from X to ℓ:** The distance from a point (x,y,z) to the line ℓ (which is parallel to the z-axis and passes through (−d,0,0)) is given by: dist(X,ℓ)=(x+d)2+y2
4. **Calculate the distance from X to P:** The distance from a point (x,y,z) to the point P=(0,0,0) is: dist(X,P)=x2+y2+z2
5. Set up the inequality: (x+d)2+y2≥2x2+y2+z2
6. Square both sides to eliminate the square roots: (x+d)2+y2≥4(x2+y2+z2)
7. Expand and simplify the inequality: x2+2dx+d2+y2≥4x2+4y2+4z2 d2+2dx≥3x2+3y2+4z2 3x2+3y2+4z2−2dx≤d2
8. **Complete the square for the x term:** 3(x2−32dx)+3y2+4z2≤d2 3(x−3d)2−3d2+3y2+4z2≤d2 3(x−3d)2+3y2+4z2≤34d2
9. Divide through by 3: (x−3d)2+y2+34z2≤94d2
10. Recognize the equation of an ellipsoid: The inequality represents an ellipsoid centered at (3d,0,0) with semi-axes 32d, 32d, and 3d.
11. Calculate the volume of the ellipsoid: The volume V of an ellipsoid with semi-axes a, b, and c is given by: V=34πabc Here, a=32d, b=32d, and c=3d.
12. **Substitute the values of a, b, and c:** V=34π(32d)(32d)(3d) V=34π⋅94d2⋅3d V=34π⋅934d3 V=27316πd3
The final answer is 27316πd3
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