This question is incomplete, the complete question is;
find the critical points and classify them as local maxima, local minima, saddle points, or none of these.
f(x,y) = (x + y)(xy + 1)
Answer:
(x,y) = (-1, 1), (1, -1) area critical points
f(xx) =2y, fyy =2x,f(xy) =2x + 2y, D = f(xx)fyy - f(xy² )
at (-1, 1)
f(xx) = 2 ,fyy =-2,f(xy) = 0, D = -4 < 0 saddle point
at (1, -1)
f(xx) = -2, fyy =2,f(xy) =0, D = -4 < 0 saddle point
Explanation:
Given that;
f(x,y) = (x + y)(xy + 1)
f(x,y) =x²y + xy² + x + y
for critical points fx =0 ,fy =0
fx = 2xy + y² + 1 = 0, fy = x² + 2xy + 1 = 0
2xy + y² + 1 = 0, x²+ 2xy + 1 = 0
2xy + y² + 1 - x² - 2xy - 1 = 0
x² = y²
=> x = y, x = -y
2xy + y² + 1 = 0, x = y
2yy + y² + 1 = 0
3y² = -1 , no solution
2xy + y² + 1 = 0, x = -y
-2yy + y² + 1 = 0
=> -y2 + 1 = 0
=> y = -1, y = 1
y = -1 => x = 1, y = 1 => x = -1
(x,y) = (-1, 1), (1, -1) area critical points
f(xx) =2y, fyy =2x,f(xy) =2x + 2y, D = f(xx)fyy - f(xy² )
at (-1, 1)
f(xx) = 2 ,fyy =-2,f(xy) = 0, D = -4 < 0 saddle point
at (1, -1)
f(xx) = -2, fyy =2,f(xy) =0, D = -4 < 0 saddle point