Solve[(1/R1+G/R2)/(1/R1+1/R2+1/Ri)==-G/A,G] 

                             A R2 Ri
Out[1]= {{G -> -(-------------------------------)}}
                 R1 R2 + R1 Ri + A R1 Ri + R2 Ri

R1/(1+G/A) /. First[%]                                                  

Simplify[%] 

        R2 Ri + R1 (R2 + Ri + A Ri)
Out[3]= ---------------------------
              R2 + Ri + A Ri

R1+Ri R2/(1+A)/(Ri+R2/(1+A))                                            

Simplify[%]

        R2 Ri + R1 (R2 + Ri + A Ri)
Out[5]= ---------------------------
              R2 + Ri + A Ri
