# Thread: using f(t) = 3t/(t+3), what is the velocity of a train at t = 7

1. ## using f(t) = 3t/(t+3), what is the velocity of a train at t = 7

For some reason, this is one of only two questions I'm stumped on right now.

I've calculated the distance @ 21/10. But when I follow the examples in Stewart Calculus ed:7 I can't work out how to calculate the velocity.

Time = 7, distance = 21/10. I'm not even close to the answer, or even sure how I'm supposed to go about the problem.

Not asking for it to be done, just a step in the right direction.

2. ## Re: using f(t) = 3t/(t+3), what is the velocity of a train at t = 7

If f(t) is the distance function then f'(t) is the velocity function. Take the derivative and evaluate it when t = 7

Please post your calculations so we can see where you need help.

3. ## Re: using f(t) = 3t/(t+3), what is the velocity of a train at t = 7

cheers, I missed the concept for this one early on and was off on a tangent.

Sometimes these are so easy once you know what to do.

I basically did [g(f')- f(g')] / g^2 plugging in values and it was straight forward.

ended up with 9/100.