Pretty easy relative max problem calc 3

In summary, the conversation discusses finding critical points for a function and the correct formula for calculating d, which involves the mixed second derivative. The conversation also clarifies that the mixed second derivative can be found in two different ways.
  • #1
Mdhiggenz
327
1

Homework Statement



Locate all relative max,min,and saddle points if any.

f(x,y)=x2+xy+y2-3x

fx=2x+y-3

fy=x+2y

Skipping some algebra I get the critical points (2,-1)

fxx=2
fyy=2

d=fxx*fyy-f(x0,y0)2

d=4-9=-5

I know I'm messing up at f(x0,y0)

I'm simply plugging in my c.p points (2,-1) into

f(x,y)=x2+xy+y2-3x

but seem to keep getting -3

While the value 1 is suppose to come out

Homework Equations





The Attempt at a Solution

 
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  • #2
Mdhiggenz said:

Homework Statement



Locate all relative max,min,and saddle points if any.

f(x,y)=x2+xy+y2-3x

fx=2x+y-3

fy=x+2y

Skipping some algebra I get the critical points (2,-1)

fxx=2
fyy=2

d=fxx*fyy-f(x0,y0)2

d=4-9=-5

I know I'm messing up at f(x0,y0)

I'm simply plugging in my c.p points (2,-1) into

f(x,y)=x2+xy+y2-3x

but seem to keep getting -3

While the value 1 is suppose to come out

Homework Equations





The Attempt at a Solution


It looks like you have your formula for d wrong. It's supposed to be d=fxx*fyy-(fxy)^2. fxy is the mixed second derivative.
 
  • #3
I'm a bit confused what do you mean mixed second derivative?
 
  • #4
Mdhiggenz said:
I'm a bit confused what do you mean mixed second derivative?

First take the derivative of f with respect to x to get fx. Then take the derivative of fx with respect of y to get fxy. That's the mixed second derivative. Or you can do it in the other order and get fyx. They should be the same.
 
Last edited:
  • #5
Got it, thanks Dick.
 

Related to Pretty easy relative max problem calc 3

What is a relative maximum in Calculus 3?

A relative maximum is a point on a graph where the function has the highest value within a certain interval. This means that there are points both to the left and right of the maximum where the function has a lower value.

How do you find the relative maximum of a function in Calculus 3?

To find the relative maximum of a function, you must first take the derivative of the function and set it equal to 0. Then, solve for the variable to find the critical points. Next, plug the critical points into the original function to determine the values at those points. The highest value among the critical points is the relative maximum.

Can a function have more than one relative maximum?

Yes, a function can have multiple relative maxima. This occurs when the function has multiple peaks within an interval. Each peak will have its own relative maximum value.

What is the difference between a relative maximum and an absolute maximum?

A relative maximum is a point where the function has the highest value within an interval, while an absolute maximum is the highest value of the entire function. A relative maximum may occur at a critical point, but an absolute maximum can only occur at an endpoint of the interval or at a point where the derivative is undefined.

Why is it important to find relative maxima in Calculus 3?

Finding relative maxima is important in Calculus 3 because it allows us to determine important information about the function, such as the behavior and rate of change. It also helps us to identify important points on the graph and can be useful in optimization problems.

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