# Why only shared resistances are taken into consideration while computing Elmore delay?

While we compute the delay , using Elmore delay model we take into consideration the shared resistance and capacitance. I would like to know why are we only concerned with shared resistance not the other resistances.

($$\ A \cap B \$$ means the set that contains all those elements that A and B have in common.)
The solution in the slide is: $$\tau_{Di} = \sum^{N}_{k=1} C_k \cdot R_{ik} =$$ $$C_1 \cdot R_1 + C_2 \cdot R_1 + C_3 \cdot (R_1+R_3) + C_4 \cdot (R_1+R_3) + C_i \cdot (R_1+R_3+R_i)$$
Note that using the definition given on the wiki, "summing the delays through each segment as the R (electrical resistance) times the downstream C", gives the same result. $$R_1 \cdot (C_1+C_2+C_3+C_4+C_i) + R_3 \cdot (C_3+C_4+C_i) + R_i \cdot C_i$$