1/A = 1/A1 + 1/A2
What does A equal to?
If you really mean $\displaystyle \frac{1}{A}= \frac{1}{A_1}+ \frac{1}{A_2}$, where A, $\displaystyle A_1$, and $\displaystyle A_2$ refer to different numbers, then go ahead and add the fractions on the right: $\displaystyle \frac{1}{A}= \frac{A_1+ A_2}{A_1A_1}$.
Now "invert" both fractions:
$\displaystyle A= \frac{A_1A_2}{A_1+ A_2}$