Rates of change (1 Viewer)

[dolbinau]

You know that extension change thing e.g see 2005 HSC 7 b)ii)

How do I know how to set it up?

What I mean is, we're given dA/dt (let A = alpha)

But couldn't it be

dA/dt=(dr/dt)(dA/dr)

and

dr/dt=(dA/dt)(dr/dA)

So, like I said, how do I know how to actually set it up? In the answers it's the second one but I just don't know why

thanks

 

[dolbinau]

But then we have to find dA/dr which I wouldn't know how to do unless we change the subject of the equation in the previous Q. - in that case does it work just as well?