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Unformatted text preview: HW 6 Solutions Math 1920 Spring 2009 March 3, 2009 Problem 14.8.1 Let g ( x,y ) = x 2 + 2 y 2 1 so that our constraint is g = 0. Then f = y i + x j and g = 2 x i + 4 y j . Setting f = g gives the equations y = 2 x and x = 4 y . Substituting we get x = 8 x 2 , so either x = 0 or = 2 4 . If x = 0, then y = 0, but (0,0) is not on the ellipse, so we can not have x = 0. Therefore = 2 4 , so x = 2 y . Substituting this into g ( x,y ) = 0 gives 2 y 2 + 2 y 2 1 = 0, so y = 1 2 So the extreme values of f on the ellipse occur at 2 2 , 1 2 and 2 2 , 1 2 . The extreme values of f on the ellipse are 2 4 . Problem 14.8.13 Let f ( x,y ) = x 2 + y 2 and g ( x,y ) = x 2 2 x + y 2 4 y . Then f = 2 x i + 2 y j and g = (2 x 2) i + (2 y 4) j . Setting f = g gives the equations 2 x = (2 x 2) and 2 y = (2 y 4) . Solving we get x = 1  1 and y = 2  1 for 6 = 1. If = 1 we have 2 x = 2 x 2, which is impossible. Therefore y = 2 x . Substituting this into g ( x,y ) = 0 gives 0 = x 2 2 x + 4 x 2 8 x = 5 x 2 10 x = 5 x ( x 2). So either x = 0, in which case y = 0, or x = 2, in which case y = 4. The minimum value of f on the circle is f (0 , 0) = 0 and the maximum value is f (2 , 4) = 20. Problem 14.8.17 We will minimize the square of the distance from the points on the plane to (1,1,1). The square the distance to (1,1,1) is given by the function f ( x,y,z ) = ( x 1) 2 +( y 1) 2 +( z 1) 2 . Let g ( x,y,z ) = x +2 y +3 z 13. Then f = (2 x 2) i + (2 y 2) j + (2 z 2) k and g = i + 2 j + 3 k . Setting f = g gives the equations 2 x 2 = , 2 y 2 = 2 , and 2 z 2 = 3 . Therefore x = +2 2 = y +1 2 and z = 3 +2 2 = 3 y 1 2 ....
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 Spring '06
 PANTANO
 Equations, Multivariable Calculus

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