Question

A wire that is 40 cm long is bent into the shape of a rectangle whose width is x cm.

(i) Find an expression, in terms of x, for the area, A cm², of the rectangle. Find the x-intercepts on the graph of A against x.
(ii) Find the x-intercepts on the graph of A against x.
(iii) Find the maximum area that can be formed.
(iv) Show that this maximum area is only possible if the shape formed is a square.

How do I solve (iii) and (iv)? Thank you!​

Answer 1

Answer:(iii) To find the maximum area of the rectangle, we can use the formula for the area of a rectangle, A = lw, where l is the length and w is the width. Since the wire is 40 cm long, we have l = 40 cm - x cm = 40 - x cm. The maximum area occurs when the length and width are equal, so we set l = w and solve for x:

40 - x = x

2x = 40

x = 20

So, the maximum area of the rectangle is A = lw = (40 - x)x = (40 - 20) * 20 = 20 * 20 = 400 cm².

(iv) To show that the maximum area is only possible if the shape formed is a square, we use the result from part (iii) that the maximum area occurs when the width x = 20 cm. If the width is less than 20 cm, the length will be greater than 40 - x, and the area will be smaller. If the width is greater than 20 cm, the length will be less than 40 - x, and the area will be smaller. So, the maximum area is only possible if x = 20 cm, which means the rectangle is a square.

Explanation: