# Solve for the specified variable?

• Mar 4th 2011, 10:29 AM
mrmyers22
Solve for the specified variable?
What do I do, I'm the HORRIBLE when it comes to math :'( I have a lot more of these but i need to learn how to do this.

1/a + 1/b = c for a

P= A/1+rt for r

I= kE/R for R
• Mar 4th 2011, 11:00 AM
Quacky
$\frac{1}{a}+\frac{1}{b}=c$

Start by removing the fractions (just multiply through by the denominators, as below):

$a(\frac{1}{a}+\frac{1}{b})=a\times c$

$1+\frac{a}{b}=ac$

$b(1+\frac{a}{b})=ac\times b$

$b+a=abc$

Rearrange so that all terms involving 'a' are on one side of the equation:

$b=abc-a$

Factor out an $a$:

$b=a(bc-1)$

Divide through by the non-a factor:

$\frac{b}{bc-1}=a$

Try the others yourself, it's the same principle! Post your working if you're still unable to continue.
• Mar 4th 2011, 11:56 AM
Wilmer
Quote:

Originally Posted by mrmyers22
1/a + 1/b = c for a

Multiply each term by ab:
b + a = abc
Get the terms with "a" together:
abc - a = b
factor left side:
a(bc - 1) = b
divide each side by (bc - 1):
a = b / (bc - 1)