Question

please, remind me definition when it is local max, local min or saddle point. second derivative must be what? i remember there is huge rule. please, write the rule or give me link on it. (talking about surface where function is f(x,y)

Answer 1

http://www.cliffsnotes.com/study_guide/Second-Derivative-Test-for-Local-Extrema.topicArticleId-39909,articleId-39893.html

local max would have the second derivative - at a critical point (because the first derivative is decreasing from + to -). local min would have second derivative + at a critical point (because the first derivative is increasing from - to +). This doesn't help with the saddle point, but according to

http://mathworld.wolfram.com/SecondDerivativeTest.html

you can use the second derivative test discriminant to determine saddles.

Answer 2

second link-is what i need) ty!

Answer 3

my pleasure