# maximize and minimize

• Apr 14th 2009, 01:33 PM
Abbas
maximize and minimize
A wire 20 m long is cut into two pieces. One piece is bent in the shape of
an equilateral triangle, and the other is bent in the shape of a circle. How
should the wire be cut to maximize the total area enclosed by the shapes?
How should it be cut to minimize the total area?

any help?
i got complicated in the numbers :S
root 3 and pi !!

regards
• Apr 14th 2009, 03:08 PM
skeeter
Quote:

Originally Posted by Abbas
A wire 20 m long is cut into two pieces. One piece is bent in the shape of
an equilateral triangle, and the other is bent in the shape of a circle. How
should the wire be cut to maximize the total area enclosed by the shapes?
How should it be cut to minimize the total area?

any help?
i got complicated in the numbers :S
root 3 and pi !!

regards

$x$ = perimeter of triangle ... each side $s = \frac{x}{3}$

area of an equilateral triangle ...

$A = \frac{\sqrt{3}}{4}s^2 = \frac{\sqrt{3}}{4}\left(\frac{x}{3}\right)^2$

$20-x$ = circumference of circle

$2\pi r = 20-x$

$r = \frac{20-x}{2\pi}$

$A = \pi\left(\frac{20-x}{2\pi}\right)^2$

total area ...

$A = \pi\left(\frac{20-x}{2\pi}\right)^2 + \frac{\sqrt{3}}{4}\left(\frac{x}{3}\right)^2$

find $\frac{dA}{dx}$ and optimize
• Apr 15th 2009, 02:19 PM
Abbas
u'll get one value for x only , so how it will be cut to maximize if the value was local minimum ,?
• Apr 15th 2009, 02:54 PM
skeeter
Quote:

Originally Posted by Abbas
u'll get one value for x only , so how it will be cut to maximize if the value was local minimum ,?

consider the endpoint extrema.
• Apr 16th 2009, 02:32 PM
Abbas
what the domain will be?
is it from [0,20]?
• Apr 16th 2009, 02:44 PM
skeeter
Quote:

Originally Posted by Abbas
what the domain will be?
is it from [0,20]?

yes ...

x = 0 , all circle

x = 20 , all triangle