Question

A piece of wire 18cm long is bent to form a rectangle. If its length is x cm, obtain an expression for its area in terms of

* and hence calculate the dimensions of the rectangle with maximum area

Answer 1

To form a rectangle, the piece of wire will have two sides of length x and two sides of length (18 - 2x)/2 = 9 - x. Therefore, the perimeter of the rectangle is given by:

2x + 2(9 - x) = 18 - 2x

The area of the rectangle is given by:

A = x(9 - x)

Expanding this expression, we get:

A = 9x - x^2

To find the dimensions of the rectangle with maximum area, we can differentiate the area expression with respect to x:

dA/dx = 9 - 2x

Setting this equal to zero to find the maximum:

9 - 2x = 0

x = 4.5

So, one side of the rectangle is x = 4.5 cm and the other side is (18 - 2x)/2 = 4.5 cm. Therefore, the dimensions of the rectangle with maximum area are 4.5 cm by 4.5 cm.

To calculate the maximum area, we can substitute x = 4.5 into the area expression:

A = 9(4.5) - (4.5)^2 = 20.25 cm^2

Explanation: