Answer:
Part A)
The lower base has a radius of 9 units, and the upper base has a radius of 6 units.
Part B)
5 units.
Part C)
75π or about 235.62 square units.
Step-by-step explanation:
Please refer to the figure below.
The cone has a radius of 9 units and a total height of 12 units.
A frustum of the cone has a height of 4 units.
Part A)
The lower radius of the frustum will simply be 9 units.
For the upper radius, we will use the properties of similar triangles. We will compare the smaller upper triangle to the overall larger triangle. This yields:
[tex]\displaystyle \frac{12}{8}=\frac{9}{x}[/tex]
Solve for x. Simplify:
[tex]\displaystyle \frac{3}{2}=\frac{9}{x}[/tex]
Cross-multiply:
[tex]18=3x\Rightarrow x=6[/tex]
The upper base has a radius of 6 units.
Part B)
We can first find the total slant height of the entire cone. By using the Pythagorean Theorem, this yields that the total slant height is:
[tex]SH^2=12^2+9^2[/tex]
Simplify:
[tex]SH^2=225\Rightarrow SH=15\text{ units}[/tex]
Now, find the slant height of the upper cone:
[tex](SH_\text{cone})^2=8^2+6^2=100[/tex]
So:
[tex]SH_\text{cone}=10[/tex]
Then the slant height of the frustum will be the cone subtracted from the total. Thus:
[tex]SH_{\text{frustum}}=15-10=5\text{ units}[/tex]
Part C)
We can first find the lateral area of the entire cone. The lateral area is given by:
[tex]LA=\pi r\ell[/tex]
The lateral area of the entire cone will be:
[tex]LA=\pi (9)(15)=135\pi[/tex]
The lateral area of the upper cone will be:
[tex]LA_\text{cone}=\pi(6)(10)=60\pi[/tex]
Then the lateral area of the frustum is:
[tex]LA_\text{frustum}=135\pi-60\pi =75\pi\text{ units}^2\approx235.62\text{ units}^2[/tex]
