# Dimensions

• Nov 29th 2008, 09:12 AM
>_<SHY_GUY>_<
Dimensions
I have a few quick questions for this project i have...

5. if you wanted the area to be 20 m^2, what would the dimensions of a rectangle to minimize the amount of fencing needed? use a graphing calculator to determine the answer.

-->i remember him saying that the the dimensions is related to the square root of 20. i just don't remember how to get the formula to put it in the calculator and show it...

the whole project's purpose is to show him our knowledge to solve real-life problems that can be analyzed by Graphical, Numerical or Algebraic. [Numerical meaning completing a table of values]

and one question asks: what are the weaknesses and strengths of these strategies?

any help would be appreciated...thank you!
• Nov 29th 2008, 09:17 AM
running-gag
Let x and y be the dimensions of the rectangle
Area is xy=20
Perimeter is P=2(x+y)=2(x+20/x)
Differentiate P gives P'(x)=2(1-20/x²)
P'(x)=0 for x=square root of 20
• Nov 29th 2008, 09:19 AM
>_<SHY_GUY>_<
Quote:

Originally Posted by running-gag
Let x and y be the dimensions of the rectangle
Area is xy=20
Perimeter is P=2(x+y)=2(x+20/x)
Differentiate P gives P'(x)=2(1-20/x²)
P'(x)=0 for x=square root of 20

thats where i got lost...how did you get x^2 in your denominator?
im trying to work out the problem but i know im doing something wrong
• Nov 29th 2008, 09:23 AM
running-gag
Remember that differentiate 1/x gives -1/x² (Wink)
• Nov 29th 2008, 09:27 AM
>_<SHY_GUY>_<
Quote:

Originally Posted by running-gag
Remember that differentiate 1/x gives -1/x² (Wink)

Oh(Surprised)... Sorry!
Thank you!