c)
\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}
\frac{1}{R}=\frac{R_1+R_2}{R_1*R_2}
R(R_1+R_2)=R_1R_2/:(R_1+R_2)
R=\frac{R_1R_2}{R_1+R_2}
…
\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}
\frac{1}{R}=\frac{R_1+R_2}{R_1*R_2}
R(R_1+R_2)=R_1R_2
RR_1+RR_2=R_1R_2
RR_2-R_1R_2=-RR_1/*(-1)
R_1R_2-RR_2=RR_1
R_2(R_1-R)=RR_1/:(R_1-R)
R_2=\frac{RR_1}{R_1-R}