2.2 #35

1 / R = 1 / R₁ + 1 / R₂      solve for R₁

-1 / R₁ = 1 / R₂ - 1 / R 

-1 / R₁ (RR₁R₂) = 1 / R₂ (RR₁R₂) - 1 / R (RR₁R₂)

-RR₂ = RR₁ - R₁R₂

-RR₂ = R₁(R - R₂)

{ R₁ = - RR₂ / R - R₂ }