Question 1016551
*[tex \large \frac{1}{F} = (n-1)\left(\frac{1}{R_1} + \frac{1}{R_2}\right)]

Since *[tex \large \frac{1}{R_1} + \frac{1}{R_2} = \frac{R_1 + R_2}{R_1R_2}] this is equivalent to 


*[tex \large \frac{1}{F} = (n-1)\left(\frac{R_1 + R_2}{R_1R_2}\right) = \frac{(n-1)(R_1 + R_2)}{R_1R_2}]


Invert both sides to get the desired answer:


*[tex \large F = \frac{R_1R_2}{(n-1)(R_1 + R_2)]