# Rearrange of formula

1. Jan 29, 2013

### rad10k

1. The problem statement, all variables and given/known data
I am trying to rearrange formula to make r^2 the subject.

2. Relevant equations

0.05=(2.88/4*1*0.018)(0.05^2-r^2)
3. The attempt at a solution
I tried to rearrage as :

0.05^2+0.05/2.88/4*0.018 then sqr root of the answer but I dont think it is correct
Please help

2. Jan 29, 2013

### haruspex

There's at least one mistake, maybe more - can't be sure because you've left out parentheses.
Please post your individual individual steps, using parentheses as appropriate.

3. Jan 29, 2013

### rad10k

Thank you for your reply,

(0.05^2+0.05)/(2.88/4*0.018)=r^2 this is what I had changed it to ....
thanks

4. Jan 29, 2013

### haruspex

That looks worse!
You start with 0.05=(2.88/4*1*0.018)(0.052-r2). Do you mean 0.05=(2.88/(4*1*0.018))(0.052-r2) or 0.05=((2.88/4)*1*0.018)(0.052-r2)?
Whichever, post your individual steps, just making one simple change at a time.

5. Jan 30, 2013

### rad10k

Ok So I started with :

0.05=(2.88/4*1*0.018)(0.05^2-r^2)

I wish to make r^2 the subject so ,

I rearrange to make ;

r^2 = (0.05^2+0.05) / (2.88/4*1*0.018)

As the first two parentheses are multiply I change it to a divide.

I exchange r^2 with the 0.05 and used a plus instead of a minus in the formula.

6. Jan 30, 2013

### tiny-tim

hi rad10k!

some people can do these things safely in their head

some people can't

you can't (you get confused about where to put the brackets)

you need to follow haruspex's advice …

just do one operation at a time​

(and not just this question, but always, including in the exam, or you'll make mistakes and lose marks!)

7. Jan 30, 2013

### rad10k

This is the equation :

u={∆P/4.Lμ}(R^2-r^2)
and I wish to find r^2 but a unsure how to rearrange it I thought maybe;

r^2=u/{∆P.Lμ}(R^2 ) ??

8. Jan 30, 2013

### tiny-tim

why are you not following our advice?

do it in steps, ie one operation at a time!

the first step would be

u = {∆P/4.Lμ}(R2-r2)

u/{∆P/4.Lμ} = (R2-r2)​

9. Jan 31, 2013

### rad10k

u = {∆P/4.Lμ}(R2-r2)

u/{∆P/4.Lμ} = (R2-r2)

This was arrived by dividing both sides by {∆P/4.Lμ} and cancelling on the right to leave (R2-r2) on its own .

So then do I need to subtract R2 from both sides cancelling out R2 on the right leaving;

u/{∆P/4.Lμ} - R2 = -r2 ?

I apologise but I am rubbish at rearranging

10. Jan 31, 2013

### haruspex

But see how much better you do when you take smaller steps!

11. Feb 1, 2013

### tiny-tim

then you need to practise!

(caution: you'll always be a bit rubbish,

so no matter how much practice you get, and how good you think you've become,

you must always do it one step at a time, so that in the pressure of the exam:

i] you're less likely to make a mistake

ii] if you do make a mistake, at least the examiner can see it's a silly error, and only take off one mark! )

12. Feb 1, 2013

### rad10k

Thanks guys :)

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