Among all rectangles that have a perimeter of 156, find the dimensions of the one whose area is largest. write your answers as fractions reduced to lowest terms.

Question

Kendrick Ibarra

Mathematics

2 May, 07:21

Among all rectangles that have a perimeter of 156, find the dimensions of the one whose area is largest. write your answers as fractions reduced to lowest terms.

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Gwendolen · Answer 1

P = 2L + 2W

We have a perimeter of 156, so we have

2L + 2W = 156

Let the length = x

2x + 2W = 156

The width is

2W = 156 - 2x

W = 78 - x

The area of a rectangle is A = LW

A = x (78 - x)

A = 78x - x^2

This is an inverted parabola, so there is a maximum value.

78x - x^2 = 0

x (78 - x) = 0

x = 0 or x = 78

The zeros of the parabola are at x = 0 and x = 78.

Since the parabola is symmetric over its vertical axis, the maximum values occurs at the x-value in the middle of 0 to 78, which is 39.

At x = 39, the area has a maximum value.

L = 39 & W = 39

It's a square with side measuring 39.