Posted by **REALLY NEED HELP!!!** on Monday, November 1, 2010 at 5:46pm.

A fence is to be built to enclose a rectangular area of 230 square feet. The fence along three sides is to be made of material that costs 5 dollars per foot, and the material for the fourth side costs 15 dollars per foot. Find the dimensions of the enclosure that is most economical to construct.

- Math: Calculus -
**REALLY NEED HELP!!!**, Monday, November 1, 2010 at 5:50pm
My dimensions that I got is sqrt(215.625)*230/sqrt(215.625) but it's wrong.

- Math: Calculus -
**Damon**, Monday, November 1, 2010 at 5:53pm
cost = 5(2b+L) + 15 L = 10 b + 20 L

bL = 230 so b = 230/L

cost = 2300/L + 20 L

d cost/dL = -2300/L^2 + 20 L

= 0 for min

20 L = 2300/L^2

L ^3 = 115

L = 4.86

b = 47.3

check my arithmetic !!!

- error -
**Damon**, Monday, November 1, 2010 at 5:55pm
cost = 5(2b+L) + 15 L = 10 b + 20 L

bL = 230 so b = 230/L

cost = 2300/L + 20 L

d cost/dL = -2300/L^2 + 20 *******

= 0 for min

20 = 2300/L^2

L ^2 = 115

L = 10.72

b = 21.5

b = 47.3

- Math: Calculus -
**bobpursley**, Monday, November 1, 2010 at 5:59pm
cost=5*(2w+L)+15L

Area= Lw or w= ARea/L=230/L

cost= 10(230/L)+5L+15L

dcost/dL= -2300/L^2+20=0

L=sqrt 115 and w= 230/sqrt115

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