Question 1043662
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Let L be the length of the rectangle, and W be the width.


Then L + W + L + W = P, perimeter, or  2*(L+W) = P,  or  L + W = {{{P/2}}} = {{{80/2}}} = 40 feet.

Hence, W = 40 -L.

The area of the rectangle is equal to 

A = L*W = L*(40-L) = {{{40L - L^2}}}.

So, A = {{{40L - L^2}}} expresses the area of the rectangle as the function of L.
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Solved.


You can find many simple solved introductory problems on area of rectangle in  the lesson 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Surface-area/Problems-on-the-area-and-the-perimeter-of-a-rectangle.lesson>Problems on the area and the dimensions of a rectangle</A>

in this site.