Math Problem Statement

The surface area of sphere of radius 5cm, is five times the are of the curved surface area of a cone of radius 4cm. Find the height and volume of the cone.

Solution

To solve this, we’ll start by calculating the surface area of the sphere and then use the relationship to determine the cone’s height and volume.

1. Surface Area of the Sphere:

The formula for the surface area of a sphere is:

Asphere=4πr2A_{\text{sphere}} = 4 \pi r^2

For a sphere of radius r=5cmr = 5 \, \text{cm}:

Asphere=4π(5)2=100πcm2A_{\text{sphere}} = 4 \pi (5)^2 = 100 \pi \, \text{cm}^2

It is given that this is five times the curved surface area of the cone.

2. Curved Surface Area of the Cone:

The formula for the curved surface area of a cone is:

Acone, curved=πrlA_{\text{cone, curved}} = \pi r l

where:

  • rr is the radius of the cone base (r=4cmr = 4 \, \text{cm}),
  • ll is the slant height of the cone.

Let the curved surface area of the cone be xx. From the problem:

100π=5xx=100π5=20π100 \pi = 5 \cdot x \quad \Rightarrow \quad x = \frac{100 \pi}{5} = 20 \pi

Thus, the curved surface area of the cone is 20πcm220 \pi \, \text{cm}^2.

Substitute Acone, curved=20πA_{\text{cone, curved}} = 20 \pi into the formula:

20π=π(4)ll=20π4π=5cm20 \pi = \pi (4) l \quad \Rightarrow \quad l = \frac{20 \pi}{4 \pi} = 5 \, \text{cm}

So, the slant height of the cone is l=5cml = 5 \, \text{cm}.

3. Height of the Cone:

Using the Pythagorean theorem, we find the height of the cone (hh):

l2=r2+h252=42+h2l^2 = r^2 + h^2 \quad \Rightarrow \quad 5^2 = 4^2 + h^2

25=16+h2h2=9h=9=3cm25 = 16 + h^2 \quad \Rightarrow \quad h^2 = 9 \quad \Rightarrow \quad h = \sqrt{9} = 3 \, \text{cm}

So, the height of the cone is h=3cmh = 3 \, \text{cm}.

4. Volume of the Cone:

The formula for the volume of a cone is:

[ V = \frac{1}{3

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Math Problem Analysis

Mathematical Concepts

Geometry
Surface Area
Volume
Pythagorean Theorem

Formulas

Surface area of a sphere: 4πr^2
Curved surface area of a cone: πrl
Pythagorean theorem: l^2 = r^2 + h^2
Volume of a cone: V = (1/3)πr^2h

Theorems

Pythagorean Theorem

Suitable Grade Level

Grades 8-10