Homework Help: Rate of Increase Question

1. Jun 12, 2010

Procrastinate

The radius of a circular ink spot, t seconds after it first appears, is given by:

$$r=\frac{1+4t}{2+t}$$

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?

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2. Jun 12, 2010

hgfalling

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.