Partial derivative problem... why is my answer wrong?

  • #1

Homework Statement


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.

Homework 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)

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.
upload_2017-11-28_22-25-24.png
 

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Answers and Replies

  • #2
Delta2
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I also get 3.697... but where did u get the minus sign from, you input the velocities as negatives?
 
  • #3
I also get 3.697... but where did u get the minus sign from, you input the velocities as negatives?
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!

Thanks
 
  • #4
PeroK
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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!

Thanks
I agree with your answer.
 
  • #5
Ray Vickson
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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!

Thanks
More simply: the two people get closer together as ##t## increases.
 
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  • #6
DrClaude
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Have you tried fewer decimals? Four significant digits is a bit much considering the numbers given.
 
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  • #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.
 
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  • #8
DrClaude
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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.
And I suggested you use fewer :rolleyes:
 

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