Math Help - Please help me with this Chain Rule and Cosine theorem problem

1. Please help me with this Chain Rule and Cosine theorem problem

Hi All,

I have the next problem to solve and I am trying to solve it for 2 days with no correct result. I used Chain Rule and I got:

but the answer is still not correct.

Please someone if can see let me know where is my mistake

Thank you

2. Re: Please help me with this Chain Rule and Cosine theorem problem

note that $\frac{df}{dt} < 0$ because the distance between the two is decreasing.