# Math Help - subject of formula

1. ## subject of formula

1/A = 1/A1 + 1/A2

What does A equal to?

2. Originally Posted by the undertaker
1/A = 1/A1 + 1/A2

What does A equal to?
Here's a start:

The common denominator for the right hand side is $A_1 A_2$:

$\frac{1}{A} = \frac{A_2}{A_1 A_2} + \frac{A_1}{A_1 A_2} = \frac{A_2 + A_1}{A_1 A_2}$.

3. Originally Posted by the undertaker
1/A = 1/A1 + 1/A2

What does A equal to?
If you really mean $\frac{1}{A}= \frac{1}{A_1}+ \frac{1}{A_2}$, where A, $A_1$, and $A_2$ refer to different numbers, then go ahead and add the fractions on the right: $\frac{1}{A}= \frac{A_1+ A_2}{A_1A_1}$.

Now "invert" both fractions:
$A= \frac{A_1A_2}{A_1+ A_2}$