MATH 105 Lecture Notes - Lecture 7: Maxima And Minima
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Math 105 lecture 7 partial derivatives and maximum/minimum problems con"t. Definitions of local extrema and theorem 12. 13 (12. 8, p 939) A function has a local maximum value at (a,b) if f(x,y) = f(a,b) for all (x,y) in the domain of f in some open disk centered at (a,b). A function f has a local minimum value at (a,b) if f(x,y) >/= f(a,b) for all (x,y) in the domain of f in some open disk centered at (a,b). Local maximum and local minimum values are also local extreme values or local extrema. If f has a local maximum or minimum value at (a,b) and the partial derivatives fx and fy exist at (a,b), then fx(a,b) = fy(a,b) = 0. Suppose f has a local maximum value at (a,b). the function of one variable g(x) = f(x,b), obtained by holding y=b fixed, also has a local maximum at (a,b). However, g"(a) = fx(a,b); therefore fx(a,b) = 0.