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Sagot :
Answer:
a) f(x,y) = - 1 minimum at P ( 0 ; -1 )
b) f (x,y) = 10 maximum at P ( 3 , 1 ) and f (x,y) = - 10 minimum at Q ( - 3 , - 1 )
c) Max f ( x , y ) = 2 for points P ( 1, 2 ) and T ( -1 , -2 )
Min f ( x , y ) = -2 for points Q ( 1 , - 2 ) and R ( -1 , 2 )
Step-by-step explanation:
A) f(x,y) = x² - y² subject to x² + y² = 1 g(x,y) = x² + y²- 1
δf(x,y)/ δx = 2*x δg(x,y)/ δx = 2*x
δf(x,y)/ δy = - 2*y δg(x,y)/ δy = 2*y
δf(x,y)/ δx = λ* δg(x,y)/ δx
2*x = λ*2*x
δf(x,y)/ δy = λ* δg(x,y)/ δy
- 2*y = λ*2*y
Then, solving
2*x = λ*2*x x = λ*x λ = 1
- 2*y = λ*2*y y = - 1
x² + y²- 1 = 0 x² + ( -1)² - 1 = 0 x = 0
Point P ( 0 ; -1 ) ; then at that point
f(x,y) = x² - y² f(x,y) = 0 - ( -1)² f(x,y) = - 1 minimum
b) f( x, y ) = 3*x + y g ( x , y ) = x² + y² = 10
δf(x,y)/ δx = 3 δg(x,y)/ δx = 2*x
δf(x,y)/ δy = 1 δg(x,y)/ δy = 2*y
δf(x,y)/ δx = λ * δg(x,y)/ δx ⇒ 3 = 2* λ *x (1)
δf(x,y)/ δy = λ * δg(x,y)/ δy ⇒ 1 = 2*λ * y (2)
x² + y² - 10 = 0 (3)
Solving that system
From ec (1) λ = 3/2*x From ec (2) λ = 1/2*y
Then (3/2*x ) = 1/2*y 3*y = x
x² + y² = 10 ⇒ 9y² + y² = 10 10*y² = 10
y² = 1 y ± 1 and
y = 1 x = 3 P ( 3 , 1 ) y = - 1 x = -3 Q ( - 3 , - 1 )
Value of f( x , y ) at P f (x,y) = 3*x + y f (x,y) = 3*(3) +1
f (x,y) = 10 maximum at P ( 3 , 1 )
Value of f( x , y ) at Q f (x,y) = 3*x + y f (x,y) = 3*(- 3) + ( - 1 )
f (x,y) = - 10 minimum at Q ( - 3 , - 1 )
c) f( x, y ) = xy g ( x , y ) = 4*x² + y² - 8
δf(x,y)/ δx = y δg(x,y)/ δx = 8*x
δf(x,y)/ δy = x δg(x,y)/ δy = 2*y
δf(x,y)/ δx = λ * δg(x,y)/ δx ⇒ y = λ *8*x (1)
δf(x,y)/ δy = λ * δg(x,y)/ δy ⇒ x = λ *2*y (2)
4*x² + y² - 8 = 0 (3)
Solving the system
From ec (1) λ = y/8*x and From ec (2) λ = x/2*y Then y/8*x = x/2*y
2*y² = 8*x² y² = 4*x²
Plugging that value in ec (3)
4*x² + 4*x² - 8 = 0
8*x² = 8 x² = 1 x ± 1 And y² = 4*x²
Then:
for x = 1 y² = 4 y = ± 2
for x = -1 y² = 4 y = ± 2
Then we get P ( 1 ; 2 ) Q ( 1 ; - 2)
R ( - 1 ; 2 ) T ( -1 ; -2)
Plugging that values in f( x , y ) = xy
P ( 1 ; 2 ) f( x , y ) = 2 R ( - 1 ; 2 ) f( x , y ) = - 2
Q ( 1 ; - 2) f( x , y ) = -2 T ( -1 ; -2 ) f( x , y ) = 2
Max f ( x , y ) = 2 for points P and T
Min f ( x , y ) = -2 for points Q and R
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