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Homework Help: Partial derivative problem... why is my answer wrong?

  1. Nov 29, 2017 #1
    1. The problem statement, all variables and given/known data
    The entire problem is in the attached picture. I have been checking and double checking for about an hour, found solutions online which agree with my solution, but I cannot find any answer beside -3.697 m/s which is marked wrong by the computer program.

    2. Relevant equations
    Is the homework program wrong or am I somehow missing something?

    df/dt = (2x-y)/(2(x^2+y^2-xy)^(1/2))(dx/dt) + (2y-x)/(2*(x^2+y^2 -xy)^(1/2))(dy/dt)

    3. The attempt at a solution
    All of this works out to -3.697. I have tried rounding, leaving the answer as postive, nothing seems to work. Very frustrated.
  2. jcsd
  3. Nov 29, 2017 #2


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    I also get 3.697... but where did u get the minus sign from, you input the velocities as negatives?
  4. Nov 29, 2017 #3
    The minus sign comes from the fact that the distances x and y at time t will be given by x = 21 - 5t and y = 25 - 3t hence dx/dt = -5 and dy/dt = -3. But the computer will not accept it either way. Unless any one else can see some mistake we've both made, I will now go complain to my teacher!

  5. Nov 29, 2017 #4


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    I agree with your answer.
  6. Nov 29, 2017 #5

    Ray Vickson

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    More simply: the two people get closer together as ##t## increases.
  7. Nov 29, 2017 #6


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    Staff: Mentor

    Have you tried fewer decimals? Four significant digits is a bit much considering the numbers given.
  8. Nov 30, 2017 #7
    It turns out that whoever wrote my homework program has never in their life heard of significant figures. Given the values involved the answer should contain only 1 sigfig, but the solution was to input SIX MORE DECIMAL PLACES from my calculator.
  9. Dec 1, 2017 #8


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    Staff: Mentor

    And I suggested you use fewer :rolleyes:
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