R(r1+r2) = r1r2, For r2
\(R(r_1+r_2)=r_1r_2\\ Rr_1+Rr_2=r_1r_2\\ Rr_2-r_1r_2=-Rr_1\\ r_2(R-r_1)=-Rr_1\\ r_2=-\dfrac{Rr_1}{R-r_1}\)