Question

A box with a square base and open top must have a volume of 32,000 cm3. find the dimensions of the box that mini- mize the amount of material used.

Answer 1

alrighty


squaer base so length=width, nice


v=lwh
but in this case, l=w, so replace l with w
V=w²h

and volume is 32000
32000=w²h


the amount of materials is the surface area
note that there is no top
so
SA=LW+2H(L+W)
L=W so
SA=W²+2H(2W)
SA=W²+4HW

alrighty

we gots
SA=W²+4HW and
32000=W²H

we want to minimize the square foottage
get rid of one of the variables
32000=W²H
solve for H
32000/W²=H
subsitute

SA=W²+4WH
SA=W²+4W(32000/W²)
SA=W²+128000/W

take derivitive to find the minimum
dSA/dW=2W-128000/W²
where does it equal 0?

0=2W-1280000/W²
128000/W²=2W
128000=2W³
64000=W³
40=W

so sub back
32000/W²=H
32000/(40)²=H
32000/(1600)=H
20=H

the box is 20cm height and the width and length are 40cm