Wire Problem - A Precalculus Approach

There are many ways that students may approach this problem. Consider selecting and sequencing techniques when having students present their work. In precalculus, a graphing approach is ideal.

Overall, this problem consists of selecting where to cut the wire, constructing a square from one piece and a circle from the other, and computing the area of each shape.

Let $x$ be the distance from the left end of the wire as shown below.

Cutting here will result in two pieces of wire, one with length $x$ and the other with length $40-x$. Let the piece of wire with length $x$ be used to construct the circle and the piece of wire with length $40-x$ be used to construct the square.

For the circle, the length of the wire $x$ corresponds to the circumference of the circle. That is, $x=2\pi r$. To determine the area of the circle, we must first solve for $r$

$x = 2\pi r$

$\frac{x}{2\pi} = r$

Now we find the area of the circle using $A=\pi r^2$

$A_{circle}=\pi \left( \frac{x}{2\pi} \right) ^2$

$A_{circle}=\frac{x^2}{4\pi}$

For the square, the length of the wire $40-x$ corresponds to its perimeter. That is, $P=40-x$. To calculate the area of the square, we must first determine the side length of the square. Since a square has 4 congruent sides, we have:

$4s = 40-x$

$s=\frac{40-x}{4}$

Now we find the area of the square using $s^2$

$A_{square}=\left( \frac{40-x}{4} \right) ^2$

The total combined area can be found by adding the two areas

$A=\frac{x^2}{4\pi}+\left( \frac{40-x}{4} \right) ^2$

Creating a Table

Using the area formula above, students may consider a guess-and-check method by trying out different values $x$. A table of values for $x$ and its corresponding areas can be constructed.

Using the Grapher app, input functions to compute the area of the circle, the area of the square, and the total area. Then view the Table tab and input possible values for $x$.

Using this approach, students will find the maximum at $x = 40$. That is, when the entire piece of wire is used to construct a circle, the area is maximized at $127.324 \textup{cm}^2$.

Graphing

Using the area formula above, students may graph the function $f(x)=\frac{x^2}{4\pi}+\left( \frac{40-x}{4} \right) ^2$ and look for maximums.

Using the Grapher app, enter the function $f(x) = \frac{x^2}{4\pi}+\left( \frac{40-x}{4} \right) ^2$.

Before viewing the Graph, open the Function options by clicking on the three dots to the right of the expression. Adjust the plot restriction in the Function options to match the practical domain for this scenario.

Graphing the function, we see the maximum occurs at the right endpoint $\left( 40,127.324 \right)$.

That is, when the entire piece of wire is used to construct a circle, the area is maximized at $127.324 \textup{cm}^2$