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Unformatted text preview: (11/11/08) Math 10C. Lecture Examples. Section 14.4. Directional derivatives and gradient vectors in the plane Example 1 (a) Find the directional derivative of f ( x , y ) = x 2 + y 2 at ( 1 , 1 ) in the direction of the unit vector u = ( 1 2 2 , 1 2 2 ) (Figure 1). (b) Why is it plausible that the directional derivative is positive? x 1 2 y 1 2 1 u = (big 1 2 2 , 1 2 2 )big 1 s FIGURE 1 Answer: (a) D u f (1 , 1) = 2 2 (b) f ( x,y ) = x 2 + y 2 is increasing in the direction of u at (1 , 1) in Figure 1 because its graph is a circular paraboloid that opens upward. Example 2 What is the derivative of f ( x , y ) = x 2 y 5 at P = ( 2 , 1 ) in the direction toward Q = ( 4 , ) ? Answer: D u f (2 , 1) = 2 5 Example 3 What is the derivative of h ( x , y ) = e xy at (2,3) in the direction at an angle of 2 3 radians from the positive xdirection?...
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This note was uploaded on 09/30/2011 for the course MATH 10C taught by Professor Hohnhold during the Spring '07 term at UCSD.
 Spring '07
 Hohnhold
 Calculus, Derivative, Vectors

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