# Finding an unknown in a formula

• Nov 18th 2011, 03:19 AM
David Green
Finding an unknown in a formula
1 / 4/5 + 1/v = 1/f

Find the value of v

1 / 4/5 + 1/v = 1 / 2/3 - 1 / 4/5 = 1/v

1 / 2/3 - 1 / 4/5 = 5/3 - 1 x 9 / 1 x 5 = 5/3 - 9/5 =

5 x 5 / 3 x 9 = 25 / 27

I am sure I am not quite right with this?
• Nov 18th 2011, 03:44 AM
emakarov
Re: Finding an unknown in a formula
By 1 / 4/5 do you mean (a) $1\tfrac{4}{5}$, (b) $\frac{1}{4/5}$ or (c) $\frac{1/4}{5}$? These options can be rendered in text as follows: (a) 1 4/5, (b) 1 / (4 / 5) and (c) (1 / 4) / 5 (maybe also adding a note would help).

Your equation has two variables: v and f, so at most you can express v through f.

LaTeX formulas have to be surrounded by [TEX]...[/TEX] tags (that's the rightmost button over the text area where you type your post, after you press "Preview Post" button).
• Nov 18th 2011, 03:47 AM
David Green
Re: Finding an unknown in a formula
Quote:

Originally Posted by emakarov
By 1 / 4/5 do you mean (a) $1\tfrac{4}{5}$, (b) $\frac{1}{4/5}$ or (c) $\frac{1/4}{5}$? These options can be rendered in text as follows: (a) 1 4/5, (b) 1 / (4 / 5) and (c) (1 / 4) / 5 (maybe also adding a note would help).

Your equation has two variables: v and f, so at most you can express v through f.

LaTeX formulas have to be surrounded by [TEX]...[/TEX] tags (that's the rightmost button over the text area where you type your post, after you press "Preview Post" button).

I mean what you said in (b) above.

Thanks for the additional help with latex, I am sure I get it right soon.

David
• Nov 18th 2011, 04:18 AM
emakarov
Re: Finding an unknown in a formula
Since $\frac{1}{4/5}=\frac{5}{4}$, the equation is $\frac{5}{4}+\frac{1}{v}=\frac{1}{f}$.

$\frac{1}{v}=\frac{1}{f}-\frac{5}{4}=\frac{4-5f}{4f}$
$v=\frac{4f}{4-5f}$
• Nov 18th 2011, 04:50 AM
David Green
Re: Finding an unknown in a formula
Quote:

Originally Posted by emakarov
Since $\frac{1}{4/5}=\frac{5}{4}$, the equation is $\frac{5}{4}+\frac{1}{v}=\frac{1}{f}$.

$\frac{1}{v}=\frac{1}{f}-\frac{5}{4}=\frac{4-5f}{4f}$
$v=\frac{4f}{4-5f}$

Thanks for your help we have now solved it.

P.S. your PM box is full lol

David