Answer: minimum value of f = -2 and maximum value of f = 2
Step-by-step explanation:
Given that;
f(x, y) = xy; 4x² + y² = 8
S(x,y) = xy + λ(4x² + y² - 8)
dS/dλ = 4x² + y² - 8 = 0
dS/dx = y + 8λx = 0 -> y = -8λx
dS/dy = x + 2yλ = 0 -> x + 2λ(-8λx) = x(1 - 16λ²) = x(1-4λ)(1+4λ) = 0
-> x = 0 , λ = 1/4 , λ = -1/4
x = 0 -> y = 0 -> it doesn't satisfy 4x² + y² = 8
λ = 1/4 -> y = -8λx = -2x -> 4x² + y² = 8x² = 8 -> x = -1 , 1 -> y = 2 , -2
λ = -1/4 -> y = -8λx = 2x -> 4x² + y² = 8x² = 8 -> x = -1 , 1 -> y = -2 , 2
we have 4 critical points:
(-1, 2) -> f = -2
(1, 2) -> f = 2
(-1, -2) -> f = 2
(1 , -2) -> f = -2
Therefore; minimum value of f = -2 and maximum value of f = 2
