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Unformatted text preview: Version 084 Homework 12 Gilbert (59825) 1 This printout should have 11 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. The due time is Central time. Ive put a few numerical free response questions on this assignment to test this feature. Return your answer as a deci mal to, say, 3 places. For the answer to be graded as correct is has to be within 1% of the correct answer. 001 10.0 points Determine the minimum value of f ( x, y, z ) = 2 x 2 + y 2 + 2 z 2 + 2 subject to the constraint 3 x + 2 y + 2 z = 5 . 1. min value = 30 7 2. min value = 32 7 3. min value = 89 21 4. min value = 92 21 correct 5. min value = 94 21 Explanation: By the method of Lagrange Multipliers the minimum value of f ( x, y, z ) = 2 x 2 + y 2 + 2 z 2 + 2 subject to the constraint ( ) 3 x + 2 y + 2 z = 5 will occur at a critical point of the Lagrange function F ( x, y, z, ) = 2 x 2 + y 2 + 2 z 2 + 2 + (3 x + 2 y + 2 z 5) . This critical point will be the common solu tion ( x , y , z , ) of the equations F x = 4 x + 3 = 0 , F y = 2 y + 2 = 0 , F z = 4 z + 2 = 0 , and F = 3 x + 2 y + 2 z 5 = 0 . Solving, we see that x = 3 4 , y = , z = 1 2 , so 21 4 + 5 = 0 , i.e. , = 20 21 . Thus the minimum value of f subject to the constraint ( ) occurs at parenleftBig 5 7 , 20 21 , 10 21 parenrightBig . Consequently, the minimum value of f subject to the constraint ( ) is given by min value = 92 21 . 002 10.0 points The temperature T at a point ( x, y, z ) on the surface x 2 + y 2 + z 2 = 48 is given by T ( x, y, z ) = x + y + z in degrees centigrade. Find the maximum temperature on this surface. Correct answer: 12. Explanation: Version 084 Homework 12 Gilbert (59825) 2 We have to maximize the function T sub ject to the constraint x 2 + y 2 + z 2 = 48 . The corresponding Lagrange function is F ( x, y, z, ) = x + y + z + ( x 2 + y 2 + z 2 48) . At the critical point of F , therefore, F x = 1 + 2 x = 0 , F y = 1 + 2 y = 0 , F z = 1 + 2 z = 0 , F = x 2 + y 2 + z 2 48 = 0 . Thus x = y = z = radicalbig 48 / 3 = 4 ....
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 Fall '07
 Gilbert

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