Answer:
Saddle point: ![(0,0)](https://tex.z-dn.net/?f=%280%2C0%29)
Local minimum: ![(\frac{3}{8}, -\frac{3}{8})](https://tex.z-dn.net/?f=%28%5Cfrac%7B3%7D%7B8%7D%2C%20-%5Cfrac%7B3%7D%7B8%7D%29)
Local maxima:
, ![(\frac{9}{8},0)](https://tex.z-dn.net/?f=%28%5Cfrac%7B9%7D%7B8%7D%2C0%29)
Step-by-step explanation:
The function is:
![f(x,y) = 8\cdot y^{2}\cdot x -8\cdot y\cdot x^{2} + 9\cdot x \cdot y](https://tex.z-dn.net/?f=f%28x%2Cy%29%20%3D%208%5Ccdot%20y%5E%7B2%7D%5Ccdot%20x%20-8%5Ccdot%20y%5Ccdot%20x%5E%7B2%7D%20%2B%209%5Ccdot%20x%20%5Ccdot%20y)
The partial derivatives of the function are included below:
![\frac{\partial f}{\partial x} = 8\cdot y^{2}-16\cdot y\cdot x+9\cdot y](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20x%7D%20%3D%208%5Ccdot%20y%5E%7B2%7D-16%5Ccdot%20y%5Ccdot%20x%2B9%5Ccdot%20y)
![\frac{\partial f}{\partial x} = y \cdot (8\cdot y -16\cdot x + 9)](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20x%7D%20%3D%20y%20%5Ccdot%20%288%5Ccdot%20y%20-16%5Ccdot%20x%20%2B%209%29)
![\frac{\partial f}{\partial y} = 16\cdot y \cdot x - 8 \cdot x^{2} + 9\cdot x](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20y%7D%20%3D%2016%5Ccdot%20y%20%5Ccdot%20x%20-%208%20%5Ccdot%20x%5E%7B2%7D%20%2B%209%5Ccdot%20x)
![\frac{\partial f}{\partial y} = x \cdot (16\cdot y - 8\cdot x + 9)](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20y%7D%20%3D%20x%20%5Ccdot%20%2816%5Ccdot%20y%20-%208%5Ccdot%20x%20%2B%209%29)
Local minima, local maxima and saddle points are determined by equalizing both partial derivatives to zero.
![y \cdot (8\cdot y -16\cdot x + 9) = 0](https://tex.z-dn.net/?f=y%20%5Ccdot%20%288%5Ccdot%20y%20-16%5Ccdot%20x%20%2B%209%29%20%3D%200)
![x \cdot (16\cdot y - 8\cdot x + 9) = 0](https://tex.z-dn.net/?f=x%20%5Ccdot%20%2816%5Ccdot%20y%20-%208%5Ccdot%20x%20%2B%209%29%20%3D%200)
It is quite evident that one point is (0,0). Another point is found by solving the following system of linear equations:
![\left \{ {{-16\cdot x + 8\cdot y=-9} \atop {-8\cdot x + 16\cdot y=-9}} \right.](https://tex.z-dn.net/?f=%5Cleft%20%5C%7B%20%7B%7B-16%5Ccdot%20x%20%2B%208%5Ccdot%20y%3D-9%7D%20%5Catop%20%7B-8%5Ccdot%20x%20%2B%2016%5Ccdot%20y%3D-9%7D%7D%20%5Cright.)
The solution of the system is (3/8, -3/8).
Let assume that y = 0, the nonlinear system is reduced to a sole expression:
![x\cdot (-8\cdot x + 9) = 0](https://tex.z-dn.net/?f=x%5Ccdot%20%28-8%5Ccdot%20x%20%2B%209%29%20%3D%200)
Another solution is (9/8,0).
Now, let consider that x = 0, the nonlinear system is now reduced to this:
![y\cdot (8\cdot y+9) = 0](https://tex.z-dn.net/?f=y%5Ccdot%20%288%5Ccdot%20y%2B9%29%20%3D%200)
Another solution is (0, -9/8).
The next step is to determine whether point is a local maximum, a local minimum or a saddle point. The second derivative test:
![H = \frac{\partial^{2} f}{\partial x^{2}} \cdot \frac{\partial^{2} f}{\partial y^{2}} - \frac{\partial^{2} f}{\partial x \partial y}](https://tex.z-dn.net/?f=H%20%3D%20%5Cfrac%7B%5Cpartial%5E%7B2%7D%20f%7D%7B%5Cpartial%20x%5E%7B2%7D%7D%20%5Ccdot%20%5Cfrac%7B%5Cpartial%5E%7B2%7D%20f%7D%7B%5Cpartial%20y%5E%7B2%7D%7D%20-%20%5Cfrac%7B%5Cpartial%5E%7B2%7D%20f%7D%7B%5Cpartial%20x%20%5Cpartial%20y%7D)
The second derivatives of the function are:
![\frac{\partial^{2} f}{\partial x^{2}} = 0](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%5E%7B2%7D%20f%7D%7B%5Cpartial%20x%5E%7B2%7D%7D%20%3D%200)
![\frac{\partial^{2} f}{\partial y^{2}} = 0](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%5E%7B2%7D%20f%7D%7B%5Cpartial%20y%5E%7B2%7D%7D%20%3D%200)
![\frac{\partial^{2} f}{\partial x \partial y} = 16\cdot y -16\cdot x + 9](https://tex.z-dn.net/?f=%5Cfrac%7B%5Cpartial%5E%7B2%7D%20f%7D%7B%5Cpartial%20x%20%5Cpartial%20y%7D%20%3D%2016%5Ccdot%20y%20-16%5Ccdot%20x%20%2B%209)
Then, the expression is simplified to this and each point is tested:
![H = -16\cdot y +16\cdot x -9](https://tex.z-dn.net/?f=H%20%3D%20-16%5Ccdot%20y%20%2B16%5Ccdot%20x%20-9)
S1: (0,0)
(Saddle Point)
S2: (3/8,-3/8)
(Local maximum or minimum)
S3: (9/8, 0)
(Local maximum or minimum)
S4: (0, - 9/8)
(Local maximum or minimum)
Unfortunately, the second derivative test associated with the function does offer an effective method to distinguish between local maximum and local minimums. A more direct approach is used to make a fair classification:
S2: (3/8,-3/8)
(Local minimum)
S3: (9/8, 0)
(Local maximum)
S4: (0, - 9/8)
(Local maximum)
Saddle point: ![(0,0)](https://tex.z-dn.net/?f=%280%2C0%29)
Local minimum: ![(\frac{3}{8}, -\frac{3}{8})](https://tex.z-dn.net/?f=%28%5Cfrac%7B3%7D%7B8%7D%2C%20-%5Cfrac%7B3%7D%7B8%7D%29)
Local maxima:
, ![(\frac{9}{8},0)](https://tex.z-dn.net/?f=%28%5Cfrac%7B9%7D%7B8%7D%2C0%29)