# Homework Help: Rearrange of formula

1. Jan 29, 2013

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

2. Jan 29, 2013

### haruspex

There's at least one mistake, maybe more - can't be sure because you've left out parentheses.

3. Jan 29, 2013

(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!
Whichever, post your individual steps, just making one simple change at a time.

5. Jan 30, 2013

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

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)

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

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

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