# A wire of length L can be used to make a circle and a square. How much of the wire should be used for each of the figures to maximise area?

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

The total length of the wire is L. Let us use it to make a square of length x and a circle of radius r. The combined area of the shapes is x^2 + pi*r^2. The circumference of the square is 4x and that of the circle is 2*pi*r

4x + 2*pi*r = L

We have to maximize A = x^2 + pi*r^2

Differentiate L and A with respect to r

dA/ dr = 2*x(dx/dr) + 2*pi*r

dL / dr = 4(dx/ dr) + 2*pi

As L is a constant dL/dr = 0

=> 4(dx/ dr) + 2*pi = 0

=> dx / dr = -2*pi/4 = -pi/2

substitute in dA/dr

=> dA/ dr = 2*x(-pi/2) + 2*pi*r

=> dA/dr = -pi*x + 2*pi*r

Take the second derivative with respect to r

=> d^2A / dr^2 = 2*pi – pi*(dx/dr)

=> d^2A / dr^2 = 2*pi + pi^2/2

The second derivative is always positive. The function of A versus r is concave upwards.

In the interval 0 <= 2*pi*r <= L, the function A takes the maximum value at either of r = 0 or r = L or both.

At r = 0, x = L/4, we find A = L^2 / 16

At r = L/2*pi, x = 0, we have A = L^2/ 4*pi

This gives the maximum value of A at r = L/ 2*pi

Therefore we can conclude that for the maximum area the wire should be used only to make the circle and no part of it used for the square.

hala718 | High School Teacher | (Level 1) Educator Emeritus

Posted on

WE will assume that we will use x of the wire for the square and (L-x) of the wire for the circle.

Now the area of the square is given by:

A= side^2 = (x/4)^2 = x^2 /16

Now the area of the circle is given by:

A = r^2 * pi

But we have the circumference C = L-x

==> 2*pi*r = L-x

==> r = (L-x)/2pi

==>A = (L-x)^2/4*pi^2 * pi = (L-x)^2/4pi

Let us find the maximum area of the circle.

==> A' = 2(L-x)*-1 / 4pi = (x-L)/2pi = 0

==> x = L ...........(1)

For the square:

A' = 2x/16 = x/8

The maximum values is 0.

Then, the maximum area is when the wire used to make a circle only.

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