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

## 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.

#### Attachments

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Delta2
Homework Helper
Gold Member
I also get 3.697... but where did u get the minus sign from, you input the velocities as negatives?

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

PeroK
Homework Helper
Gold Member
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

Ray Vickson
Homework Helper
Dearly Missed
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.

WWGD
DrClaude
Mentor
Have you tried fewer decimals? Four significant digits is a bit much considering the numbers given.

Delta2
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.

DrClaude
DrClaude
Mentor
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