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Rearrange of formula

by rad10k
Tags: rearrange formulae
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rad10k
#1
Jan29-13, 04:59 AM
P: 61
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
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haruspex
#2
Jan29-13, 05:36 AM
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Quote Quote by rad10k View Post
0.05^2+0.05/2.88/4*0.018
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.
rad10k
#3
Jan29-13, 03:23 PM
P: 61
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

haruspex
#4
Jan29-13, 04:13 PM
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Rearrange of formula

Quote Quote by rad10k View Post
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
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.
rad10k
#5
Jan30-13, 03:22 AM
P: 61
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.
tiny-tim
#6
Jan30-13, 03:47 AM
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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!)
rad10k
#7
Jan30-13, 04:27 AM
P: 61
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 ) ??
tiny-tim
#8
Jan30-13, 04:40 AM
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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)
rad10k
#9
Jan31-13, 11:30 AM
P: 61
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
haruspex
#10
Jan31-13, 04:27 PM
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Quote Quote by rad10k View Post
I am rubbish at rearranging
But see how much better you do when you take smaller steps!
tiny-tim
#11
Feb1-13, 06:47 AM
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Quote Quote by rad10k View Post
I apologise but I am rubbish at rearranging
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! )
rad10k
#12
Feb1-13, 10:25 AM
P: 61
Thanks guys :)


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