Question

Consider a wire of length 12 ft. The wire is to be cut into two pieces of length x and 12−x. Suppose the length x is used to form a circle of radius r and the length 12−x is used to form a square with side of length s. What value of x will minimize the sum of their areas?

Answer 1

Answer:

x = 8

Step-by-step explanation:

When cutting the wire we will get two pieces

x             and          12 - x

If we build a circle whith x , the lenght of the circle will be x, and if we look at the equation for a lenght of a cicle  2*π*r = x

then r  = x/2π

and consequentely   A₁  = area of a circle  =  πr²       A₁  = π*x/2π  

A₁  = x/2

With the other piece  12  -  x     we have to make an square so wehave to divide that piece in four equal length

side of the square   =    s

s   = 1/4 ( 12 - x )    and the area is      A₂  = [1/4 ( 12 - x)]²

A₂  = ( 12 - x )²/16

Then     A₁  +  A₂  =  A(t)   and this area as fuction of x

A (x)  = x/2  +  1/16 ( 144 + x² -24x)          A (x)  =[ (8x + 144 + x² -24x)]/16

Taken derivatives in both sides

A´(x)  =  8  +  2x  - 24   = 0

   2x  -16  =  0         x  =  8    and   s =  12 - 8       s  =  4