Let fx y 3xx32y2y4 on the domain D x y 5
Let f(x, y) = 3x-x^3-2y^2+y^4 on the domain D = {(x, y)| -5<=x<= 5, -2<=y <=2}. Find all critical points and use the second derivative test to determine whether they are absolute, local maxima, minima or neither. Please show step by step.
Solution
f(x,y) = 3 - 3x2 = 0
x = ±1
x = 1 maxima
x = -1 minima
partial diff wrt y : 4y3 - -4y = 0
y = 0 , ±1
y = 0 maxima
y = 1 minima
y = -1 minima
(1,0) = 2 local maxima
(-1,1) = -1 local minima
(-1,-1) = -1 local minima
(1, 1) , (1 , -1) , (-1 , 0) neither !!!
global maxima (-5 , 2)
global minima (5 , -2)
end points ~~~
