# Thread: A function and a Hessian

1. ## A function and a Hessian

For f(x,y) = root(25-x^2-y^2) calculate:

a. the values: f(3,4)....easy! Just substitute, need no help here. And gradient f(3,4) = the partial derivatives df/dx and df/dy at the value 3,4...need help here. And finally the Hessian H (3,4) which is a matrix of second partial derivatives and the values need to be plugged in.

While calculating the partial derivatives I get strange numbers or the answer 0 in every case. I don't think the answer is a matrix of zeros but the strange numbers I get if I do something else don't make much sense either.

Try to tell me what to do.

Thanks.

2. Nevermind, the teacher changed the 25 in the function to 26 in his notes so I would get the value 1 and would be able to compute the Hessian. What a fruity teacher, I should have checked his rectified notes earlier.

3. I still need help though. What entries correspond to what in a Hessian matrix?

Does it go like this for a 2 x 2 matrix -->
x y
y y

And like this in a 3 x 3 matrix -->
x y z
y y z
z z z

??

Or in some other order?

Help, thx.