（E/R1＋r）^2*R1＝(E/R2＋r)^2*R2 这个式子,怎么化简?

E^2R1/(R1+r)^2=E^2R2/(R2+r)^2,约去E^2,得R2(R1+r)^2=R1(R2+r)^2
得 R1^2R2+2rR1R2+R2r^2=R1R2^2+2rR1R2+R1r^2
即R1R2(R1-R2)=r^2(R1-R2)
得r^2=R1R2