Answer:
[tex]2.158 \mu \Omega[/tex]
Explanation:
First of all, we start by calculating the total resistance of the combination R1 and R2.
These two resistors are connected in series, so their total resistance is given by:
[tex]R_{12}=R_1+R_2[/tex]
where
[tex]R_1=0.800 \mu \Omega[/tex] is the resistance of resistor 1
[tex]R_2=15.0\mu \Omega[/tex] is the resistance of resistor 2
Substituting,
[tex]R_{12}=0.800+15.0=15.8 \mu \Omega[/tex]
Then, we observe that the combination 1-2 and resistor 3 are connected in parallel. Therefore the total resistance of these 2 combinations is given by
[tex]\frac{1}{R}=\frac{1}{R_{12}}+\frac{1}{R_3}[/tex]
where
[tex]R_{12}=15.8\mu \Omega[/tex]
[tex]R_3=2.50\mu \Omega[/tex] is the resistance of resistor 3
Substituting,
[tex]\frac{1}{R}=\frac{1}{15.8}+\frac{1}{2.50}=0.463\\R=\frac{1}{0.463}=2.158 \mu \Omega[/tex]