You need the 2nd partial derivative test. Cant be bothered to type it all out, just look at the wikipedia entry for 'second partial derivative test'.
Im having some trouble finding the critical points and relative max/min/saddle points for some questions
1. f(x,y) = 7x^2 + 4y^2
I found the critical point to be (0,0), but Im not sure how to calculate for min/max or saddle point
2. f(x,y) = e^1/2xy
Im having trouble finding the critical points and determining whether the points are max/min or saddle
3. f(x,y) = x^4 - 8xy + 2y^2 - 3
I think the critical points are (0,0), (2,4) and (-2,-4) but im not sure, and again im having trouble determining whether the points are a max/min or saddle point.
Any help or clarification would be much appreciated. Thanks!
You need to apply the second partials test (Note that is the critical point):
Now, if:
and , then there is a minimum.
and , then there is a maximum.
, then there is a saddle point.
, then no conclusion can be drawn.
In your case, , and
So what can you conclude?
If I'm interpreting this right, the function is2. f(x,y) = e^1/2xy
Im having trouble finding the critical points and determining whether the points are max/min or saddle
Thus, and
Can you find the critical points now and determine whether there are max/min/saddle pts (refer to my explanation above)?
Your critical points are correct. Someone else asked about this here.3. f(x,y) = x^4 - 8xy + 2y^2 - 3
I think the critical points are (0,0), (2,4) and (-2,-4) but im not sure, and again im having trouble determining whether the points are a max/min or saddle point.
Any help or clarification would be much appreciated. Thanks!
Now apply the second partials test to see if its a maximum, minimum, or saddle point there.
For the first one, would it be a minimum at (0,0), because D >0 and fxx >0?
For the second one, im not sure how to solve those new equations in order to get the critical points.Thats where I am stuck.
The third one was more or less clarification on the critical points, I can figure out the max/min stuff now. Thanks.
So, maybe a little help with how to go about solving the equations in the second question? Thanks alot!