When there is a large difference in two resistor's sizes, what useful approximations can be used when considering their series and parallel combinations? (This is a handy thing to know in circuit design!)

Series circuit approximation = R1 >> R2 then R2 ≈ 0

Parallel circuit approximation = R1 << R2 then R2 ≈ 0

Explanation:

in a series circuit, if there is a large difference between two resistors then we can omit the resistor which has low resistance because the higher value resistor dominates.

R1 >> R2 then R2 ≈ 0

in a parallel circuit, if there is a large difference between two resistors then we can omit the resistor which has high resistance because the lower value resistor dominates.

R1 << R2 then R2 ≈ 0