- (a) Find the maximum value of f(x₁, x2,...,xn) = √√√x₁x₂x₂ given that x₁, x2,...,x, are positive numbers and x₁ + x₂ + + x = c, where c is a constant. (b) Deduce from part (a) that if x₁, x2,...,xn are positive numbers, then x₁ + x₂ + ... + xn n DIRVO This inequality says that the geometric mean of n num- bers is no larger than the arithmetic mean of the numbers. Under what circumstances are these two ou means equal?on sulavamentes asi to dos AT-E (a) Man √√X1 X2 X =

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Author:Erwin Kreyszig
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978
CHAPTER 14 Partial Derivatives
25. Consider the problem of minimizing the function
f(x, y) = x on the curve y² + x4 - x³ = 0 (a piriform).
(a) Try using Lagrange multipliers to solve the problem.
(b) Show that the minimum value is f(0, 0) = 0 but the
Lagrange condition Vf(0, 0) = AVg(0, 0) is not satis-
fied for any value of A.
(c) Explain why Lagrange multipliers fail to find the mini-
mum value in this case.
CAS 26. (a) If your computer algebra system plots implicitly defined
curves, use it to estimate the minimum and maximum
values of f(x, y) = x³ + y³ + 3xy subject to the con-
straint (x - 3)² + (y - 3)² = 9 by graphical methods.
(b) Solve the problem in part (a) with the aid of Lagrange
multipliers. Use your CAS to solve the equations numer-
ically. Compare your answers with those in part (a).
27. The total production P of a certain product depends on
the amount L of labor used and the amount K of capital
investment. In Sections 14.1 and 14.3 we discussed how the
Cobb-Douglas model P = bL K¹-a follows from certain
economic assumptions, where b and a are positive constants
and a < 1. If the cost of a unit of labor is m and the cost
of a unit of capital is n, and the company can spend only
p dollars as its total budget, then maximizing the produc-
tion P is subject to the constraint mL + nK= p. Show
ad that the maximum production occurs when
ilgillum ogn (1-9)
and for K =
287
L
=
ар
m
-P
Sulev moninim sai bart of
28. Referring to Exercise 27, we now suppose that the pro-
duction is fixed at bLªK¹-a Q, where Q is a constant.
What values of L and K minimize the cost function
C(L, K) = mL + nK?
SIROTIXS 50 bar
17 US-FE
1573
29. Use Lagrange multipliers to prove that the rectangle with
maximum area that has a given perimeter p is a square.
t
=
n
30. Use Lagrange multipliers to prove that the triangle with
maximum area that has a given perimeter p is equilateral.
Hint: Use Heron's formula for the area:
- y)(s — z)
x)(sy)(sz) burt 2E-TS
Milupani adi
where s = p/2 and x, y, z are the lengths of the sides.
A = √√√s(sx) (s
31-43 Use Lagrange multipliers to give an alternate solution to
the indicated exercise in Section 14.7.
31. Exercise 41
33. Exercise 43
35. Exercise 45
37. Exercise 47
39. Exercise 49
32. Exercise 42
34. Exercise 44
36. Exercise 46
38. Exercise 48
40. Exercise 50
41. Exercise 51
43. Exercise 55
44. Find the maximum and minimum volumes of a rectangular
box whose surface area is 1500 cm² and whose total edge
length is 200 cm.
45. The plane x + y + 2z = 2 intersects the paraboloid
z = x² + y² in an ellipse. Find the points on this ellipse
that are nearest to and farthest from the origin.
46. The plane 4x
-
z² = x² + y² in an ellipse.
(a) Graph the cone and the plane, and observe the elliptical
nolaupe intersection.
3y + 8z = 5 intersects the cone
(b) Use Lagrange multipliers to find the highest and lowest
nisl points on the ellipse.
42. Exercise 52
CAS 47-48 Find the maximum and minimum values of f subject to
the given constraints. Use a computer algebra system to solve
the system of equations that arises in using Lagrange multipliers.
(If your CAS finds only one solution, you may need to use addi-
tional commands.)
47. f(x, y, z) = ye¹-²; 9x² + 4y² + 36z² = 36, xy + yz = 1
48. f(x, y, z) = x+y+z; x² - y² = z, x² + z² = 4
49. (a) Find the maximum value of
f(x1, x2,...,xn)
= X1X2. Xn
given that x₁, x2, ..., x, are positive numbers and
htlg
X1 + x₂ + ... + x = c, where c is a constant.
love (b) Deduce from part (a) that if x₁, x2, ..., xn are positive
$ 60
numbers, then
X1 X2 Xn <
X₁ =
451144p
This inequality says that the geometric mean of n num-
bers is no larger than the arithmetic mean of the
numbers. Under what circumstances are these two
ai
√Σ a²
Σα
to show that
x₁ + x₂ + ... + xn
w nottul means equal?
dong sulav omonis dead to dos #1-2
1
01 igaldu Σ7-1x²
50. (a) Maximize Σ₁ x/y, subject to the constraints diod
Σ=1x? = 1 and Σ-₁ y² = 1. sitt bad of oilghtom
(b) Put
n
and yi=
bi
√Σ b²
Σ ab; < Σ a {Σ b
for any numbers a₁,..., an, bi, ..., bn. This inequality
is known as the Cauchy-Schwarz Inequality.
Transcribed Image Text:978 CHAPTER 14 Partial Derivatives 25. Consider the problem of minimizing the function f(x, y) = x on the curve y² + x4 - x³ = 0 (a piriform). (a) Try using Lagrange multipliers to solve the problem. (b) Show that the minimum value is f(0, 0) = 0 but the Lagrange condition Vf(0, 0) = AVg(0, 0) is not satis- fied for any value of A. (c) Explain why Lagrange multipliers fail to find the mini- mum value in this case. CAS 26. (a) If your computer algebra system plots implicitly defined curves, use it to estimate the minimum and maximum values of f(x, y) = x³ + y³ + 3xy subject to the con- straint (x - 3)² + (y - 3)² = 9 by graphical methods. (b) Solve the problem in part (a) with the aid of Lagrange multipliers. Use your CAS to solve the equations numer- ically. Compare your answers with those in part (a). 27. The total production P of a certain product depends on the amount L of labor used and the amount K of capital investment. In Sections 14.1 and 14.3 we discussed how the Cobb-Douglas model P = bL K¹-a follows from certain economic assumptions, where b and a are positive constants and a < 1. If the cost of a unit of labor is m and the cost of a unit of capital is n, and the company can spend only p dollars as its total budget, then maximizing the produc- tion P is subject to the constraint mL + nK= p. Show ad that the maximum production occurs when ilgillum ogn (1-9) and for K = 287 L = ар m -P Sulev moninim sai bart of 28. Referring to Exercise 27, we now suppose that the pro- duction is fixed at bLªK¹-a Q, where Q is a constant. What values of L and K minimize the cost function C(L, K) = mL + nK? SIROTIXS 50 bar 17 US-FE 1573 29. Use Lagrange multipliers to prove that the rectangle with maximum area that has a given perimeter p is a square. t = n 30. Use Lagrange multipliers to prove that the triangle with maximum area that has a given perimeter p is equilateral. Hint: Use Heron's formula for the area: - y)(s — z) x)(sy)(sz) burt 2E-TS Milupani adi where s = p/2 and x, y, z are the lengths of the sides. A = √√√s(sx) (s 31-43 Use Lagrange multipliers to give an alternate solution to the indicated exercise in Section 14.7. 31. Exercise 41 33. Exercise 43 35. Exercise 45 37. Exercise 47 39. Exercise 49 32. Exercise 42 34. Exercise 44 36. Exercise 46 38. Exercise 48 40. Exercise 50 41. Exercise 51 43. Exercise 55 44. Find the maximum and minimum volumes of a rectangular box whose surface area is 1500 cm² and whose total edge length is 200 cm. 45. The plane x + y + 2z = 2 intersects the paraboloid z = x² + y² in an ellipse. Find the points on this ellipse that are nearest to and farthest from the origin. 46. The plane 4x - z² = x² + y² in an ellipse. (a) Graph the cone and the plane, and observe the elliptical nolaupe intersection. 3y + 8z = 5 intersects the cone (b) Use Lagrange multipliers to find the highest and lowest nisl points on the ellipse. 42. Exercise 52 CAS 47-48 Find the maximum and minimum values of f subject to the given constraints. Use a computer algebra system to solve the system of equations that arises in using Lagrange multipliers. (If your CAS finds only one solution, you may need to use addi- tional commands.) 47. f(x, y, z) = ye¹-²; 9x² + 4y² + 36z² = 36, xy + yz = 1 48. f(x, y, z) = x+y+z; x² - y² = z, x² + z² = 4 49. (a) Find the maximum value of f(x1, x2,...,xn) = X1X2. Xn given that x₁, x2, ..., x, are positive numbers and htlg X1 + x₂ + ... + x = c, where c is a constant. love (b) Deduce from part (a) that if x₁, x2, ..., xn are positive $ 60 numbers, then X1 X2 Xn < X₁ = 451144p This inequality says that the geometric mean of n num- bers is no larger than the arithmetic mean of the numbers. Under what circumstances are these two ai √Σ a² Σα to show that x₁ + x₂ + ... + xn w nottul means equal? dong sulav omonis dead to dos #1-2 1 01 igaldu Σ7-1x² 50. (a) Maximize Σ₁ x/y, subject to the constraints diod Σ=1x? = 1 and Σ-₁ y² = 1. sitt bad of oilghtom (b) Put n and yi= bi √Σ b² Σ ab; < Σ a {Σ b for any numbers a₁,..., an, bi, ..., bn. This inequality is known as the Cauchy-Schwarz Inequality.
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