Originally Posted by

**Sajiko** I have a question that I am stuck on for the Related Rates problems. I did it and got a negative answer when it's suppose to be positive I did it again a little differently and still got a negative answer!! Here is the problem:

Joe is 1.8 meters tall is walking directly towards a spotlight located on the ground 20 meters away from the theatre. If Joe walks at a constant rate of 1/5 m/s find the rate at which the length of his shadow on the wall is changing when he is 6 meters from the lamp.

So the 2nd time I went through it I set it up as:

Given: dx/dt = 1/5 m/s

Find: dy/dt when x = 6

y/20 = 1.8/20-x

y = 36/(20-x)

dy/dt = [-36/(20-x)^2]**(-1)**(dx/dt) **You have to use the chain rule and you forgot about the derivation of the inner function**

dy/dt = [-36/(20-6)^2](1/5)

dy/dt = -9/245

Worked from there. First time I got an answer of -1/5, second time an answer of -9/245. But I know if he is moving towards the spotlight the answer will have to be a positive rate. I think there is something fundamentally wrong with my procedure, if anyone could show me their through process on this I would greatly appreciate it!