Solve Calculus Problem: Need Help!

[Fiorella]

Calculus problem...please help!

Homework Statement

Here is a picture of the problem: http://i3.photobucket.com/albums/y62/Phio/23.jpg"

Homework Equations

A = xy = 780

cost = 8x+2y

The Attempt at a Solution

c = 8x + (1560/x)

c'= 8 - ( 1560/(x^2) )

0 = 8 - ( 1560/(x^2) )



x^2 = 195

x= \sqrt{195}

x = 13.96 = 14


I don't know what to do from here...I still can't figure out what the answer is. I think it must be either B or D.

Please help...thank you!

 

[rock.freak667]

Find the corresponding value of y.

 

[Fiorella]

oh now I see it...it's obvious. The answer is B.

Thanks!