Suppose a sphere, cylinder, and cone all share the same radius and the cylinder and cone also share the same height, which is twice the radius. Which of the following is true about the relationship between the volume formulas for the sphere, cylinder, and cone?

1 answer

Let's denote the radius of the sphere, cylinder, and cone as \( r \), and the height of the cylinder and cone as \( h \). According to the problem, the height \( h \) is twice the radius:

\[ h = 2r. \]

Now, we can calculate the volumes of each shape using their respective volume formulas:

  1. Volume of the Sphere: The formula for the volume of a sphere is given by:

    \[ V_{sphere} = \frac{4}{3} \pi r^3. \]

  2. Volume of the Cylinder: The formula for the volume of a cylinder is:

    \[ V_{cylinder} = \pi r^2 h. \]

    Substituting \( h = 2r \):

    \[ V_{cylinder} = \pi r^2 (2r) = 2\pi r^3. \]

  3. Volume of the Cone: The volume of a cone is given by the formula:

    \[ V_{cone} = \frac{1}{3} \pi r^2 h. \]

    Again substituting \( h = 2r \):

    \[ V_{cone} = \frac{1}{3} \pi r^2 (2r) = \frac{2}{3} \pi r^3. \]

Now that we have all the volumes calculated, we can summarize them:

  • Volume of the sphere: \( V_{sphere} = \frac{4}{3} \pi r^3 \)
  • Volume of the cylinder: \( V_{cylinder} = 2 \pi r^3 \)
  • Volume of the cone: \( V_{cone} = \frac{2}{3} \pi r^3 \)

Next, let's compare the volumes:

  1. Comparing the sphere and the cylinder:

    \[ V_{sphere} = \frac{4}{3} \pi r^3, \quad V_{cylinder} = 2 \pi r^3. \] To compare them, we can express \( V_{cylinder} \) in terms of \( V_{sphere} \):

    \[ V_{cylinder} = \frac{2 \cdot 3}{3} \pi r^3 = \frac{6}{3} \pi r^3. \]

    This shows that:

    \[ V_{cylinder} > V_{sphere}. \]

  2. Comparing the sphere and the cone:

    \[ V_{sphere} = \frac{4}{3} \pi r^3, \quad V_{cone} = \frac{2}{3} \pi r^3. \] Clearly, since \( \frac{4}{3} > \frac{2}{3} \), we determine that:

    \[ V_{sphere} > V_{cone}. \]

  3. Comparing the cylinder and the cone:

    \[ V_{cylinder} = 2 \pi r^3, \quad V_{cone} = \frac{2}{3} \pi r^3. \] Again, comparing:

    \[ 2 = \frac{6}{3} > \frac{2}{3} \Rightarrow V_{cylinder} > V_{cone}. \]

Conclusion:

We have established relationships:

  • \( V_{cylinder} > V_{sphere} > V_{cone} \).

In summary, among the three volumes, the cylinder has the greatest volume, the sphere has a volume greater than the cone, and the cone has the smallest volume. Thus, the true relationship is as follows:

\[ V_{cylinder} > V_{sphere} > V_{cone}. \]