Math Problem Statement

A cylinder with radius $r$ and height $h$ is inscribed in a sphere with radius $R$. The cylinder is then cut by a plane parallel to its base, resulting in a smaller cylinder at the top and a frustum (the portion of the solid that remains) below the cut. The volume of the original cylinder is $k$ times the volume of the smaller cylinder. Find the value of $k$ in terms of the central angle $\theta$ subtended by the radius of the smaller cylinder at the center of the base of the original cylinder.

Solution

We are tasked with determining the value of kk, the ratio of the volume of the original cylinder to that of the smaller cylinder, in terms of the central angle θ\theta subtended by the radius of the smaller cylinder.

Step 1: Original setup

We have:

  • A cylinder with radius rr and height hh inscribed in a sphere with radius RR.
  • The cylinder is cut by a plane parallel to its base, producing a smaller cylinder at the top and a frustum below.

Let the smaller cylinder have a radius r1r_1 and height h1h_1, and the central angle θ\theta is subtended by the radius r1r_1 of the smaller cylinder at the base of the original cylinder.

Step 2: Geometry of the inscribed cylinder and sphere

The cylinder is inscribed in the sphere, meaning that the diagonal of the cylinder (which is the space diagonal from the center of the top to the center of the bottom) is equal to the diameter of the sphere, 2R2R. The height hh of the cylinder relates to the radius of the sphere as follows: h=2R2r2h = 2\sqrt{R^2 - r^2} where rr is the radius of the cylinder.

Step 3: Volume of the original cylinder

The volume VcylV_{\text{cyl}} of the original cylinder is given by: Vcyl=πr2hV_{\text{cyl}} = \pi r^2 h Substitute the expression for hh: Vcyl=πr22R2r2V_{\text{cyl}} = \pi r^2 \cdot 2\sqrt{R^2 - r^2} Thus, the volume of the original cylinder is: Vcyl=2πr2R2r2V_{\text{cyl}} = 2\pi r^2 \sqrt{R^2 - r^2}

Step 4: Volume of the smaller cylinder

Now, consider the smaller cylinder. Its radius is r1=rcosθ2r_1 = r \cos\frac{\theta}{2}, since the angle θ\theta subtends the radius r1r_1 at the base of the original cylinder. The height of the smaller cylinder is h1h_1, which corresponds to the height of the slice formed by the plane at height h1h_1.

From the geometry, the height of the smaller cylinder is related to the angle θ\theta by the vertical projection in the sphere, giving: h1=2R2r12=2R2r2cos2θ2h_1 = 2\sqrt{R^2 - r_1^2} = 2\sqrt{R^2 - r^2 \cos^2\frac{\theta}{2}} Thus, the volume VsmallV_{\text{small}} of the smaller cylinder is: Vsmall=πr12h1=π(rcosθ2)22R2r2cos2θ2V_{\text{small}} = \pi r_1^2 h_1 = \pi \left( r \cos\frac{\theta}{2} \right)^2 \cdot 2\sqrt{R^2 - r^2 \cos^2\frac{\theta}{2}} Simplifying: Vsmall=2πr2cos2θ2R2r2cos2θ2V_{\text{small}} = 2\pi r^2 \cos^2\frac{\theta}{2} \cdot \sqrt{R^2 - r^2 \cos^2\frac{\theta}{2}}

Step 5: Ratio of volumes

The ratio kk of the volume of the original cylinder to that of the smaller cylinder is: k=VcylVsmallk = \frac{V_{\text{cyl}}}{V_{\text{small}}} Substituting the expressions for VcylV_{\text{cyl}} and VsmallV_{\text{small}}: k=2πr2R2r22πr2cos2θ2R2r2cos2θ2k = \frac{2\pi r^2 \sqrt{R^2 - r^2}}{2\pi r^2 \cos^2\frac{\theta}{2} \cdot \sqrt{R^2 - r^2 \cos^2\frac{\theta}{2}}} Simplifying further: k=R2r2cos2θ2R2r2cos2θ2k = \frac{\sqrt{R^2 - r^2}}{\cos^2\frac{\theta}{2} \cdot \sqrt{R^2 - r^2 \cos^2\frac{\theta}{2}}}

Final Answer:

Thus, the value of kk is: k=R2r2cos2θ2R2r2cos2θ2k = \frac{\sqrt{R^2 - r^2}}{\cos^2\frac{\theta}{2} \cdot \sqrt{R^2 - r^2 \cos^2\frac{\theta}{2}}}

Would you like to explore this more or ask any questions? Here are some related problems:

  1. How does the volume of the cylinder change if the radius of the sphere is doubled?
  2. What is the relationship between the height of the cylinder and the radius of the sphere?
  3. Can we compute the surface area of the smaller cylinder?
  4. How does the frustum's volume compare to the original cylinder's volume?
  5. What happens if the plane cuts the cylinder at half its height?

Tip: In problems involving inscribed shapes, always think about how the radii and heights of the objects relate geometrically to the enclosing shape (here, the sphere).

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Math Problem Analysis

Mathematical Concepts

Geometry
Solid Geometry
Cylinders
Spheres
Volume Ratios
Trigonometry

Formulas

Volume of cylinder: V = \pi r^2 h
Height of cylinder inscribed in sphere: h = 2\sqrt{R^2 - r^2}
Volume ratio: k = \frac{\sqrt{R^2 - r^2}}{\cos^2(\theta/2) \cdot \sqrt{R^2 - r^2 \cos^2(\theta/2)}}

Theorems

Pythagorean Theorem
Trigonometric Ratios

Suitable Grade Level

College level or advanced high school (Grades 11-12)