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The rectangle in the figure has sides 2a and 2b ; the curve is an ellipse. If the figure is rotated about the dotted line it generates three solids of revolution: a cone, an ellipsoid, and a cylinder. Show that the volumes are in the ratio 1:2:3 . (See L.H. Lange, American MathematicalMonthly vol. 88 (1981), p. 339.)

Short Answer

Expert verified

The volumes are in the ratio 1:2:3 .

Step by step solution

01

The volumes are in the ratio .

The sides of the rectangle are 2a,2b.

02

Step 2: Concept of the volumes

The volumes of the cone and cylinder are straightforward, and the volume of the ellipsoid can be found by transforming into a system where it turns into a sphere.

03

Find the volume of the cylinder

The volume of the cylinder is as follows:

V1=ττ²¹22b=2ττ²¹2b

Find the volume of the ellipsoid and transform it as follows:

x=x'ay=y'bz=z'b

V2=∫x∫y∫zdx'dy'dz'a2b=a2b4ττ3=4ττ3a2b

Where in the transformed equation ellipsoid turns into a sphere of radius 1.

04

Find the volume of the cone

The volume of the cone can also be found by transforming:

V3=a2b∫o2ττ»åθ∫-11dz'∫01rdr=2ττ²¹2b∫-11dz12z2=2ττ3a2b

Origin at the center of the ellipsoid.

Thus, proving the theorem results in the following,

V2V3=2V1V3=3

Hence, the volumes are in the ratio 1:2:3 .

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