Question 50623
1/R = 1/r^1 + 1/r^2 for r^2

Multiply thru by Rr^2 to get:
r^2 = rR + R
r^2-Rr-R=0
You have a quadratic in "r":
 
r=[R+-sqrt(R^2+4R)]/2

Cheers,
Stan H.