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Geometry Difficulty 4.2 AIME Find the answer United States

Problem:
A triangle with vertices at (1003,0)(1003,0), (1004,3)(1004,3), and (1005,1)(1005,1) in the xyxy-plane is revolved all the way around the yy-axis. Find the volume of the solid thus obtained.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
Let TR2T \subset \mathbb{R}^2 denote the triangle, including its interior. Then TT's area is 5/25/2, and its centroid is (1004,4/3)(1004, 4/3), so
(x,y)Txdxdy=521004=2510 \int_{(x, y) \in T} x \, dx \, dy = \frac{5}{2} \cdot 1004 = 2510
We are interested in the volume
(x,y)T2πxdxdy \int_{(x, y) \in T} 2\pi x \, dx \, dy
but this is just 2π2510=5020π2\pi \cdot 2510 = 5020\pi.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.