A zoo is designing a giant bird cage consisting of a cylinder of radius rr feet and height hh feet with a hemisphere on top (no bottom). The material for the hemisphere costs 2020 per square foot and the material for the cylindrical sides costs 1010 per square foot; the zoo has a budget of 45004500. Find the values of rr and hh giving the birds the greatest space inside assuming the zoo stays within its budget. Note: surface of a cylinder's side 2πrh2πrh, surface of a sphere 4πr24πr2, volume of a cylinder's side πr2hπr2h, volume of a sphere 43πr343πr3

r  =  42,32 ft

h  = 84.8 ft

Step-by-step explanation:

We are going to apply Lagrange multipliers method

The greatest space   means  maximum volume

V(cage)  =  Vol. of the cylinder + volume of the hemisphere

V(cylinder )  = π*r²*h

V(sphere) = (4/3)*π*r²   ⇒  V(hemisphere) = (2/3)*π*r³

V(cage) =  π*r²*h  +  (2/3)*π*r³

Associated costs:

Costs =  cost of cylinder  + cost of hemisphere

Area of the cylinder  =  Lateral area   ( no bottom no top)

Area of the cylinder  =  2*π*r*h

Area of hemisphere  =  2*π*r²

A(r,h)  =   2*π*r*h  +  2*π*r²

C(r , h ) =  10* 2*π*r*h  + 20* 2*π*r²    C(r , h ) =  20*π*r*h  + 40*π*r²

4500 = 20*π*r*h  + 40*π*r²

20*π*r*h  + 40*π*r² -  4500  =  0         20*π*r*h  + 40*π*r² -  4500 = g(r,h)

V(cage) =  π*r²*h  +  (2/3)*π*r³

δV/δr  =  2*r*π*h  + 2*π*r²                       δg(r,h)/δr  =   20*π*h + 80*π*r

δV/δh  =  π*r²                                            δg(r,h)/δh =   20*π*r

δV/δr  = λ*  δg(r,h)/δr

2*r*π*h  + 2*π*r² =  λ* 20*π*h + 80*π*r

2*r*π* ( h + r )  =  20*π*  λ* ( h + 4*r)

r* ( h + r )  = 10*λ* ( h + 4*r)                        (1)

δV/δh  = λ*  δg(r,h)/δh

π*r²  =  20*λ*π*r        r  =  20*λ                  (2)

20*π*r*h  + 40*π*r² -  4500 = 0                 (3)

We need to sole the system of equation 1 ; 2 ; 3

r = 20*λ   plugging that value in equation  1

20*λ ( h  +   20*λ  ) =  10*λ* ( h + 4*r)  

2( h  +  20*λ  ) =   ( h + 4*20*λ)  

2*h  + 40*λ  =  h + 80*λ

h  = 40*λ

20*π*r*h  + 40*π*r² -  4500 = 0  

20*π*20*λ*40*λ  + 40*π+400λ²  - 4500  = 0

16000*π*λ²  +  16000*π*λ²  = 4500

32000*π*λ² = 4500

320*π*λ² = 4500

λ² = 4500/1004,8               λ² = 4.48          λ  = 2.12

Then

r  =  20* λ         r  =  42,32 ft

h  = 40* λ         h  = 84.8 ft