Thread: 80cm of wire cut into two pieces forming a square and a circle

Hello everyone.

Wish i could answer this question on my own,

The question is from october november 2008 AS level maths paper 1 exam paper

Question:

A wire, 80cm long is cut into two pieces. One piece is bent to form a square of side x cm and the other piece is bent to form a circle of radius r cm The total area of the square and circle is A cm^2

(i) show that A=
(pi+4)x^2 - 160x + 1600
-----------------------------
pi

I have no idea how to show that...

then the second question is just as difficult.

(ii) Given that x and r can vary, find the value of x for which A has a stationary value.

Would really love some help

Thanks
Peder

Solution:

perimeter of square =4x
perimeter of circle=2 pi r
then 4x+2 pi r=80
r=(40-2x)/pi

Area of square=
Area of circle=

Total area A=
A=
A=
A=
A=

Hello, Peder!

A wire 80cm long is cut into two pieces.
One piece is bent to form a square of side cm
and the other piece is bent to form a circle of radius cm.
The total area of the square and circle is cm³.

(i) Show that: .

slovakiamath gave an excellent derivation.

(ii) Given that and can vary,
find the value of for which has a stationary value.

Differentiate , equate to zero, and solve for