A point near the cone's base

A point is chosen uniformly at random (by volume) inside a solid cone of base radius 11 and height 22 (base at the bottom, apex at the top). What is the probability the point lies within height 11 of the base?

Show hints (2)+
  1. Cross-section area at height yy is (1y/2)2\propto (1-y/2)^2; integrate for volume below height 11 over total.
  2. P=1(112)3/1=118\mathbb{P}=1-(1-\tfrac12)^3 / 1 = 1 - \tfrac18.

Answer

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0.875 (± 0.001)

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Asked at: Jane Street, DRW

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