transpose formula

Question

a little stuck on a formula 

 

1/r=1/r1+1/r2 

 

solve for r1 

Answer 1

1/r=1/r1+1/r2

1/r  - 1/r1  = 1/r2

-1/r1 = 1/r2  -  1/r

-1 = (r1)(1/r2  -  1/r)

-1/[(1/r2 - 1/r)] = r1

 

 

Answer 2

1/r = 1/r₁ + 1/r₂

 

Multiply by the LCD, rr₁r₂:  r₁r₂ = rr₂ + rr₁

 

                                        r₁r₂ - rr₁ = rr₂

 

                                        r₁(r₂ - r) = rr₂

 

                                                 r₁ = rr₂ / (r₂ - r)