one surface as follows:
then:
Set these two equations to zero and solve to find the extremum. Once you have it you can test if it is max, min, inflection or unkown by finding D:
Then, if:
one surface as follows:
then:
Set these two equations to zero and solve to find the extremum. Once you have it you can test if it is max, min, inflection or unkown by finding D:
Then, if: