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A gardener purchases a ceramic planter in the shape of a hemisphere for a small batch of leftover annuals. The volume of a hemisphere is modeled by the function [tex]V=\frac{2}{3} \pi r^3[/tex].

1. Write a model for the radius as a function of the volume.
2. The label on the planter says that it holds approximately 134 cubic inches of potting soil. What is the radius of the planter, rounded to the nearest inch? Use 3.14 for [tex]\pi[/tex].


Sagot :

Certainly! Let's tackle the problem step-by-step.

### Step 1: Write a model for the radius as a function of the volume.

We start with the volume equation of a hemisphere:
[tex]\[ V = \frac{2}{3} \pi r^3 \][/tex]

We need to solve this equation for [tex]\( r \)[/tex].

First, isolate [tex]\( r^3 \)[/tex] by multiplying both sides of the equation by [tex]\( \frac{3}{2\pi} \)[/tex]:
[tex]\[ V \cdot \frac{3}{2\pi} = r^3 \][/tex]

Now, take the cube root of both sides to solve for [tex]\( r \)[/tex]:
[tex]\[ r = \left( \frac{3V}{2\pi} \right)^{\frac{1}{3}} \][/tex]

This gives us the model for the radius as a function of the volume:
[tex]\[ r = \left( \frac{3V}{2\pi} \right)^{\frac{1}{3}} \][/tex]

### Step 2: Calculate the radius of the planter

Given:
- Volume [tex]\( V = 134 \)[/tex] cubic inches
- Use [tex]\( \pi = 3.14 \)[/tex]

Plug the given values into the radius model:
[tex]\[ r = \left( \frac{3 \cdot 134}{2 \cdot 3.14} \right)^{\frac{1}{3}} \][/tex]

First, calculate the fraction inside the parentheses:
[tex]\[ \frac{3 \cdot 134}{2 \cdot 3.14} = \frac{402}{6.28} \approx 63.98 \][/tex]

Next, take the cube root of this value:
[tex]\[ r = 63.98^{\frac{1}{3}} \approx 4.000265 \][/tex]

### Rounding to the nearest inch:

The radius, when rounded to the nearest inch, is:
[tex]\[ r \approx 4 \][/tex]

Therefore, the radius of the planter is approximately 4 inches.