Pearson Geometry Common Core, 2011
PG
Pearson Geometry Common Core, 2011 View details
3. Surface Areas of Pyramids and Cones
Continue to next subchapter

Exercise 40 Page 714

Revolving a point about a line will get us a circle.

Solid: A composite solid made by subtracting a cone from a cylinder. The radius of the cylinder is 3 units and the height is 4 units.
Surface Area: 48Ď€ square units

Practice makes perfect

We will start by analyzing the given triangle.

Let's revolve the given triangle about the line y=3. Remember that revolving a point about the line will get us a circle.

We see that the revolved triangle creates a composite solid made by subtracting a cone from a cylinder. The radius of the cylinder is 3 units and the height is 4 units. We are asked to find the surface area of the revolved solid. First, let's find the slant height of the cone, l.

We will find l using the Pythagorean Theorem for the right triangle.
r^2+ h^2= l^2
3^2+ 4^2= l^2
â–Ľ
Solve for l
9+16= l^2
25= l^2
l^2=25
l=sqrt(25)
l=5
The surface area of the composite solid consists of three parts.
  • The lateral area of the cone, L.A._(cone).
  • The lateral area of the cylinder, \text{L.A.}_\text{cyl}.
  • The base area of the cylinder, B.

Let's find them!

Radius r= 3
Height h= 4
Slant Height l= 5
\text{L.A.}_\text{cone} π r l=π ( 3)( 5)=15π
\text{L.A.}_\text{col} 2Ď€ r h=2Ď€ ( 3)( 4)=24Ď€
B π r^2=π ( 3)^2=9π

Finally, we will find the surface area of the composite solid, S.A.. \begin{gathered} \textcolor{darkviolet}{\text{S.A.}}=\text{L.A.}_\text{cone}+\text{L.A.}_\text{col}+B \\ \Downarrow \\ \textcolor{darkviolet}{\text{S.A.}}=\textcolor{darkorange}{15\pi}+\textcolor{darkorange}{24\pi}+\textcolor{darkorange}{9\pi}=\textcolor{darkviolet}{48\pi} \end{gathered} The surface area of the revolved solid is 48Ď€ square units.