Question

A box of volume 180 m3 with square bottom and no top is constructed out of two different materials. The cost of the bottom is $40/m2 and the cost of the sides is $30/m2 . Find the dimensions of the box that minimize total cost. (Let s denote the length of the side of the square bottom of the box and h denote the height of the box.)

Answer 1

Answer:

Side of square bottom=6.46 m

Height of box=4.31 m

Step-by-step explanation:

Let s be the side bottom

Height of box=h

Volume of box=$180m^3$

Volume of box=$lbh=s^2h$

$s^2h=180$

$h=\frac{180}{s^2}$

Cost of bottom=$40 per square  m

Cost of sides =$30 per square  m

Total cost=$C=40s^2+30(4sh)$

$C=40s^2+120s\times \frac{180}{s^2}$

$C(s)=40s^2+\frac{21600}{s}$

Differentiate w.r.t s

$C'(s)=80s-\frac{21600}{s^2}$

$C'(s)=0$

$80s-\frac{21600}{s^2}=0$

$80s=\frac{21600}{s^2}$

$s^3=\frac{21600}{80}=270$

$s=(270)^{\frac{1}{3}}=6.46$

Again,differentiate w.r.t s

$C''(s)=80+\frac{43200}{s^3}$

Substitute s=6.46

$C''(6.46)=80+\frac{43200}{(6.46)^3}=240.2>0$

Hence, the cost is minimum at s=6.46

$h=\frac{180}{s^2}=\frac{180}{(6.46)^2}=4.31$

Side of square bottom=6.46 m

Height of box=4.31 m