Question

1

r
=
1
r1
+
1
r2
for r2

Answer 1

do you mean

1/r = 1/r1 + 1/r2 ? If so,

1/r2 = 1/r - 1/r1 = (r1-r)/(r*r1)
so,
r2 = r*r1/(r1-r)