Question 351564
What is r: 
1/r=a/r1+b/r2 
--------------
I assume the "1" and "2" are subscripts.
---
1/r = (ar2+br1)/(r1r2)
---
Invert to get:
---
r = (r1r2)/(ar2+br1)
===========================
Cheers,
Stan H.