Question 37068
Yes, this one relates three resistance in an electrical circuit... 
Rt=1/((1/R1)+(1/R2)+(1/R3))
Just working on the right side, combine the denominators and invert (because of the one over above all of them) and get
R-sub-t = [(R-sub-1)(R-sub-2)(R-sub-3)] / [(R-sub-2)(R-sub-3) + (R-sub-1)(R-sub-3) + (R-sub-2)(R-sub-3)]