Question

The length of a rectangle is nine inches more than its width. Its area is 486 square inches. Find the width and length of the rectangle.

Answer 1

• width: 18 in
• length: 27 in

Explanation:

The relations between length (L) and width (W) are ...

W +9 = L

LW = 486

Substituting gives ...

(W+9)W = 486

W^2 +9W -486 = 0 . . . put in standard form

(W +27)(W -18) = 0 . . . . factor

W = 18 . . . . the positive solution

The width of the rectangle is 18 inches; the length is 27 inches.

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Comment on factoring

There are a number of ways to solve quadratics. Apart from using a graphing calculator, one of the easiest is factoring. Here, we're looking for factors of -486 that have a sum of 9.

486 = 2 × 3^5, so we might guess that the factors of interest are -2·3² = -18 and 3·3² = 27. These turn out to be correct: -18 +27 = 9; (-18)(27) = -486.