(15C) Parallel Resistance, two resistors

[J-F Garnier]

(08-24-2023 01:36 AM)Joe Horn Wrote:  Both programs above return 5.999999999 for values of 15 and 10 in R1 and R2 respectively. The program below returns the correct result of 6.

RCL 1
RCL × 2
RCL 1
RCL + 2
÷
RTN

Beside the two mentioned equivalent formulae:
   Req=R1.R2/(R1+R2)
and
   Req=1/(1/R1+1/R2)
there is a third (also equivalent) one that I'm using occasionally, even without programming:
   Req=R1/(1+R1/R2)
which is quite convenient when R1 is a multiple of R2, e.g.
R1=100k, R2=10k, Req=100k/(1+10) = 9.09k

The benefit of the two last formulae is that they can be easily extended to handle multiple parallel resistors:
   Req=1/(1/R1+1/R2+1/R3+...+1/Rn)
or
   Req=R1/(1+R1/R2+R1/R3+...+R1/Rn)

J-F