Расчет: Pbh3Rb - 1

R  = 
R1+R2+(R1+R2)
R = R1+R2+(R1+R2)$$R$$ = $$R1+R2+(R1+R2)$$
R =  2* R1+ 2* R2$$R$$ = $$2\cdot R1+2\cdot R2$$