Question 78666
{{{Rt=1/((1/R1)+(1/R2))}}}
This is called a "complex fraction." To simplify it, multiply
the numerator and the denominator by the LCD of the fractions
in the denominator, i.e. {{{1/R1}}} and {{{1/R2}}}. "But wait
a minute," you might say, "those are variables. How do I get an LCD?"
Well think about when you had to add {{{1/3+1/2}}}. The LCD in this
case is the product of the 3 and 2, or 6. Use the same thinking here
with the variable R1 and R2. The LCD is {{{R1*R2}}}. To maintain the
equality of the overall fraction, you need to multiply the numerator
and denomenator of the complex fraction by the same number:
{{{Rt=((R1*R2)*1)/((R1*R2)*(1/R1+1/R2))}}}
Multiplying this out, you get:
{{{Rt=((R1*R2)/((R1*R2)/R1+(R1*R2)/R2)))}}}
It is legal to cancel parts of the fraction in the denominator. 
Cancelling the R1 and the R2 gives you:
{{{Rt=((R1*R2)/(R2+R1))}}}
And you are done.

Good Luck,
tutor_paul@yahoo.com