Problem is 
Science and medicine. The combined resistance 
of two resistors R1 and R2 in a
parallel circuit is given by the formula 



Write the  on top as  to
make everything fractions:



The LCD of all the denominators is .

Write that as 

Multiply top and bottom of right side by that



The top just becomes 



In the bottom we distribute the 
into the right parentheses:



The R1's cancel in the first term on the bottom and the
R2's cancel in the second term on the bottom, so we end 
up with just



You might want to reverse the the terms in the bottom
so they will be in numerical order:



which says:

"The total resistance of two resistors in parallel is
the product of their resistances over their sum."

Edwin