Answer:
\W = Width
L = Length
Draw rectangle W x L
Divde rectangle into thirds with two lines parallel with width.
Outside fance perimeter = 2 W + 2 L
Inside fance length = 2 W
Total cost :
( 2 W + 2 L ) * 10 $ + 2 W * 20 $ = 400 $
( 2 W + 2 L ) * 10 + 2 W * 20 = 400
20 W + 20 L + 40 W = 400
60 W + 20 L = 400 Divide both sides by 20
60 W / 20 + 20 L / 20 = 400 / 20
3 W + L = 20 Subtract 3 W to both sides
3 W + L - 3 W = 20 - 3 W
L = 20 - 3W
Area :
A = W * L =
W * ( 20 - 3W ) =
20 W - 3 W ^ 2
First derivation :
dA / dW = 20 - 3 * 2 * W = 20 - 6 W
If first derivation = 0 a function has a local maximum or a local minimum.
dA / dW = 0
20 - 6 W = 0 Add 6 W to both sides
20 - 6 W + 6 W = 0 + 6 W
20 = 6 W Divide both sides by 6
20 / 6 = 6 W / 6
20 / 6 = W
2 * 10 / ( 2 * 3 ) = W
10 / 3 = W
W = 10 / 3 ft
If second derivative < 0 then function has a maximum.
If second derivative > 0 then function has a mimum.
In this case second derivative = - 6
Second derivative < 0 so function has a maximum.
For W = 10 / 3 ft
L = 20 - 3W = 20 - 3 * 10 / 3 = 20 - 10 = 10 ft
Amax = W * L = 10 / 3 * 10 = 100 / 3 ft ^2
