Maximizing the area

1. Mar 14, 2009

wcbryant87

1. The problem statement, all variables and given/known data
A running track consists of a rectangle with a semicircle at each end. If the perimeter is to be exactly 440 yards, find the dimensions (x and r) that maximize the area of the rectangle. [Hint The perimeter is 2x + 2$$\pi$$r

2. Relevant equations

3. The attempt at a solution
Ok I attempted this twice and got the exact same answer, twice. Here is what I did.

First I set up the equation: 440 = 2x + 2$$\pi$$r

I then set up the equation: Area (total) = $$\pi$$r2 + 2rx where x is the length of the side of the field (not counting the semicircles) and r is the radius.

I solved for x from the first equation and came up with x = 220 - $$\pi$$r

I then plugged the value of x into the second equation. Once I destrubuted it, I took the derivative and set it to zero and had 2r$$\pi$$ + 440 - 4$$\pi$$r = 0

Solving for r, I got 70.03. The answer in the back of the book is 110. What am I doing wrong?

I appreciate the help!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 14, 2009

tiny-tim

Hi wcbryant87!
erm … wrong area!

3. Mar 14, 2009

wcbryant87

haha. wow. the 'aha' moment has hit me. thanks!