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Optimization Problems with Functions of Two Variables

Several optimization problems are solved and detailed solutions are presented. These problems involve optimizing functions in two variables using first and second order partial derivatives.

Problems with Detailed Solutions

Problem 1

You decide to build a box that has the shape of a rectangular prism with a volume of 1000 cubic centimeters. Find the dimensions x, y, and z of the box so that the total surface area of all 6 faces of the box is minimum.
Solution to Problem 1:
The total area A of all six faces of the prism is given by:
A=2xy+2yz+2zx
optimization problem 1, minimize area

The volume of the box is given; hence
xyz=1000
Solve the above for z:
z=1000xy
Substitute z in the expression of the area A to obtain:
A(x,y)=2xy+2y(1000xy)+2x(1000xy)=2xy+2000x+2000y
We now need to find x and y that minimize the area A. We first need to find the critical points and then test the second partial derivatives. The first order partial derivatives of A are given by:
Ax(x,y)=2y2000x2 Ay(x,y)=2x2000y2
The critical points are found by setting Ax(x,y)=0 and Ay(x,y)=0 and solving the system obtained, which gives:
2y2000x2=0 2x2000y2=0
Solve the above to obtain:
x=10andy=10
We now need to find the second order partial derivatives:
Axx(x,y)=4000x3 Ayy(x,y)=4000y3 Axy(x,y)=2
We now need to test the values of Axx, Ayy, and Axy at the point (10,10) in order to use the theorem on minima and maxima of functions with 2 variables.
D=Axx(10,10)Ayy(10,10)A2xy(10,10)=4×44=12
D is positive and Axx(10,10)=4 is positive, therefore the area A is minimum for
x=10cm y=10cm z=1000xy=10cm

Problem 2

Find the dimensions of a six-faced box that has the shape of a rectangular prism with the largest possible volume that you can make with 12 squared meters of cardboard.

Solution to Problem 2:
Using all available cardboard to make the box, the total area A of all six faces of the prism is given by:
A=2xy+2yz+2zx=12

optimization problem 2, maximize area

The volume V of the box is given by:
V=xyz
Solve the equation 2xy+2yz+2zx=12 for z:
z=6xyx+y
Substitute z in the expression of the volume V to obtain:
V(x,y)=xy(6xy)x+y
Let us find the critical points by first finding the first order partial derivatives:
Vx(x,y)=y2x2+2xy6(x+y)2 Vy(x,y)=x2y2+2xy6(x+y)2
We now solve the system of equations given by Vx=0 and Vy=0. One obvious solution is given by the point (0,0) but is not physically possible. Other solutions are found by setting:
x2+2xy6=0 y2+2xy6=0
Subtracting the equations term by term we obtain:
x2y2=0
Solve to obtain:
x=yandx=y
The solution x=y is not valid for this problem since both x and y are dimensions and cannot be negative. Use x=y in the equation x2+2xy6=0, we obtain:
x2+2x26=0
Solve for x
x=2
Find y
y=x=2
Let us now find the second order partial derivatives:
Vxx(x,y)=2y2y2+6(x+y)3 Vyy(x,y)=2x2x2+6(x+y)3 Vxy(x,y)=2xyx2+3xy+y26(x+y)3
We now need the values of Vxx, Vyy, and Vxy to find the value of D=Vxx(2,2)Vyy(2,2)V2xy(2,2) in order to use the theorem on minima and maxima of functions with 2 variables.
D=Vxx(2,2)Vyy(2,2)V2xy(2,2)=52
Since D is positive and Vxx(2,2)=2 is negative, the volume V is maximum for:
x=2meters y=2meters z=6xyx+y=2meters

Problem 3

Find the distance from the point (1,2,1) to the plane given by the equation xy+z=3.

Solution to Problem 3:
One way to find the distance from a point to a plane is to take a point (x,y,z) on the plane; find the distance between this point and the given point and minimize it. Because the distance involves the square root, it would be better to minimize the square of the distance. Let the square of the distance between the given point and the point (x,y,z) on the plane be f.
f(x,y,z)=(x1)2+(y2)2+(z+1)2
We now solve the given equation xy+z=3 of the plane for z to obtain:
z=3x+y
Substitute z in f by 3x+y .
F(x,y)=(x1)2+(y2)2+(x+y+4)2
We now find the first order partial derivatives:
Fx(x,y)=2(x1)+2(1)(x+y+4) Fy(x,y)=2(y2)+2(x+y+4)
We now need to find the critical points by setting the first partial derivatives equal to zero.
2(x1)+2(1)(x+y+4)=0 2(y2)+2(x+y+4)=0
We now solve the system of equations to obtain:
(83,13,23)
We now calculate the second order derivatives:
Fxx(x,y)=4 Fyy(x,y)=4 Fxy(x,y)=2
We now need to find the sign of D=Fxx(83,13)Fyy(83,13)F2xy(83,13) in order to use the theorem on minima and maxima of functions with 2 variables
D=Fxx(83,13)Fyy(83,13)F2xy(83,13)=12
Since D is positive and Fxx is positive, F has a minimum at the point (83,13) which corresponds to a point on the plane given by
(83,13,23)
The distance d between the given point and the plane is given by
d=(18/3)2+(21/3)2+(12/3)2 =53

More on partial derivatives and multivariable functions. Multivariable Functions
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