Question

A carpenter wants to build a rectangular box with square sides in which to put round things. the material for the bottom costs $20/ft^2, material for the sides costs $10/ft^2 and the material for the top costs $50/ft^2
If the volume of the box must be 5 ft^3, then find the dimensions that will minimize the cost (and find the minimum cost).

Answer 1

Answer:$1.627\times 1.627\times 1.88\ ft^3$

Step-by-step explanation:

Given

Suppose side face have a dimension of l\times l

and width of h

volume $V=l^2\cdot h$

$h=\frac{V}{l^2}$

volume $V=5 ft^3$

Area of side wall is $A_s=l^2$

Area of top Wall $A_t=l\times h$

Area of bottom $A_b=l\times h$

Cost of bottom wall $c_b=20\times l\times h=20lh$

Cost of top wall $c_t=50\times l\times h=50lh$

Cost of side walls $c_s=4\times l^2\times 10=40l^2$

total cost $C=c_s+c_t+c_b=20lh+50lh+40l^2$

$C=70lh+40l^2$

$C=70\times l\times \frac{5}{l^2}+40l^2$

differentiate C w.r.t l to get minima or maxima

$\frac{\mathrm{d} C}{\mathrm{d} l}=0$

$-\frac{350}{l}+80l^2=0$

$l^3=\frac{350}{80}$

$l=1.627 ft$

$h=\frac{5}{2.648}$

$h=1.88 ft$

Dimension of Box is $1.627\times 1.627\times 1.88\ ft^3$