1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Rate of Increase Question

  1. Jun 12, 2010 #1
    The radius of a circular ink spot, t seconds after it first appears, is given by:

    [tex]r=\frac{1+4t}{2+t}[/tex]

    Find the rate of increase (in cm/s) at the time when th radius as doubled from its initial value.



    I have attached my working; however, the answer is 7/9cm/s and I keep getting 9/7cm/s. Does that even make sense if it is doubling anyways?
     

    Attached Files:

  2. jcsd
  3. Jun 12, 2010 #2
    I don't see any problem with your work; it seems to me that 9/7 is right. 7/9 is the rate of change at t=1, but that doesn't correspond to the condition given in the problem, since the initial radius is 1/2 and the radius at t = 1 is 5/3.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook