# Need a little help with this related rates problem

Tags:
1. Jul 12, 2016

### MrDickinson

Can someone help me with this?
(dA/dt)=1cm/s (cm^2 whatever...leave out trivial corrections).
A=pir^2
(dA/dt)=2pir(dR/dt)
Multiply through by (1/2pir)
(dA/dt)/(2pir)=dR/Dt
What is the rate of change of the radius for a circumfrance of 2
I just used the related rates formula that I derived for dR/dt to find the derivative of of the radius and I get 1/2pi, the problem is that the answer is 1/2..... gosh I suck at this. Can someone please help me?

Last edited: Jul 12, 2016
2. Jul 12, 2016

### Staff: Mentor

Taking care of units allows to avoid many mistakes in homework problems.
What did you get as value for r?

3. Jul 12, 2016

### MrDickinson

Thanks. I understand the value of checking units, but in math they are useless. I understand where you are going though, as I once had a chemistry TA make the statement "numbers without units are useless" as in... I think he was getting at something like "math without application is useless." But for me, it turns out that they usually don't do anything other than tell me what I already know; namely, I am wrong.

I think I see where I went wrong..... how stupid of me... I mean, I should feel shame and dishonor over this...

Last edited: Jul 12, 2016
4. Jul 13, 2016

### Tallus Bryne

I think you might be a bit hard on yourself. You recognized that you needed help and sought it out from those who are likely to be more knowledgeable. No one got hurt, and no one died as a result of your failure in solving this math problem. In general as well, having the humility to recognize your mistakes and actively seek to correct them is absolutely nothing to be ashamed of.

5. Jul 24, 2016

Thank you.