The formula is WRONG, the pictures name the resistance Rac, Rab, Rbc, Ra, Rb, Rc and the formula is written in terms of R1, R2,R3,Ra,Rb,Rc. See http://en.wikipedia.org/wiki/Analysis_of_resistive_circuits in the delta to wye part for show is should be. Someone please edit it. I only know the basics of wikiditing.