Natural Science Department – Duy Tan University

1. Natural Science Department – Duy Tan University MAXIMUM & MINIMUM VALUES In this section, we will learn: How to use partial derivatives to locate maxima and minima of functions of two variables Lecturer: Ho Xuan Binh Da Nang-08/2014

2. 1 MAXIMUM & MINIMUM VALUES Natural Science Department – Duy Tan University Maximum and minimum values

3. 2 Definition Natural Science Department – Duy Tan University • A function of two variables has a local maximumat (a, b) if f(x, y) ≤ f(a, b) when (x, y) is near (a, b).This means that f(x, y) ≤ f(a, b) for all points (x, y) in some disk with center (a, b). The number f(a, b) is called a local maximum value. If f(x, y) ≥ f(a, b) when (x, y) is near (a, b), then f has a local minimumat (a, b). The number f(a, b) is a local minimum value. Maximum and minimum values

4. 3 Theorem 1 Natural Science Department – Duy Tan University If f has a local maximum or minimum at (a, b) and the first-order partial derivatives of f exist there, then fx(a, b) = 0 and fy(a, b) = 0 Maximum and minimum values

5. 4 CRITICAL POINT Natural Science Department – Duy Tan University A point (a, b) is called a critical point (or stationary point) of f if either: • * fx(a, b) = 0 and fy(a, b) = 0 • * One of these partial derivatives does not exist. Maximum and minimum values

6. 5 SADDLE POINT Natural Science Department – Duy Tan University Near the origin, the graph has the shape of a saddle. • So, (0, 0) is called a saddle pointof f(x,y) = y2 – x2. Maximum and minimum values

7. 6 Theorem 2 Natural Science Department – Duy Tan University Suppose that: • The second partial derivatives of f are continuous on a disk with center (a, b). • fx(a, b) = 0 and fy(a, b) = 0 [that is, (a, b) is a critical point of f]. Let D = D(a, b) = fxx(a, b) fyy(a, b) – [fxy(a, b)]2 Maximum and minimum values

8. 6 Theorem 2 Natural Science Department – Duy Tan University • a. If D > 0 and fxx(a, b) > 0, f(a, b) is a local minimum. • b. If D > 0 and fxx(a, b) < 0, f(a, b) is a local maximum. • c. If D < 0, f(a, b) is not a local maximum or minimum. • Notes: • * In case c, The point (a, b) is called a saddle point of f . * If D = 0, the test gives no information: f could have a local maximum or local minimum at (a,b), or (a,b) could be a saddle point of f. Maximum and minimum values

9. 7 Example Natural Science Department – Duy Tan University Find the local maximum and minimum values and saddle points of f(x, y) = x4 + y4 – 4xy + 1 Maximum and minimum values

10. Thank you for your attention