Question

The unshaded area inside the figure to the right is 648 in.² Use this fact to write an equation involving x. Then solve the equation to find the value of x.
Length of rectangle is 40 inches
Width of rectangle is 22 inches
Border inside rectangle is x

Answer 1

The second order polynomial that involves the variable x (border inside the rectangle) and associated to the unshaded area is x² - 62 · x + 232 = 0.

How to derive an expression for the area of an unshaded region of a rectangle

The area of a rectangle (A), in square inches, is equal to the product of its width (w), in inches, and its height (h), in inches. According to the figure, we have two proportional rectangles and we need to derive an expression that describes the value of the unshaded area.

If we know that A = 648 in², w = 22 - x and h = 40 - x, then the expression is derived below:

A = w · h

(22 - x) · (40 - x) = 648

40 · (22 - x) - x · (22 - x) = 648

880 - 40 · x - 22 · x + x² = 648

x² - 62 · x + 232 = 0

The second order polynomial that involves the variable x (border inside the rectangle) and associated to the unshaded area is x² - 62 · x + 232 = 0. $\blacksquare$

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