Answer:
Explanation:
In the equation
[tex]\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}[/tex]we out in R = 12 and R_2 = 21 to get
[tex]\frac{1}{12}=\frac{1}{R_1}+\frac{1}{21}[/tex]Subtracting 1/21 from both sides gives
[tex]\frac{1}{12}-\frac{1}{21}=\frac{1}{R_1}[/tex]We subtract the fractions on the left-hand side to get
[tex]\frac{1\cdot21}{12\cdot21}-\frac{1\cdot12}{21\cdot12}[/tex][tex]=\frac{9}{252}=\frac{1}{28}[/tex]Hence,
[tex]\frac{1}{28}=\frac{1}{R_1}[/tex]Taking the reciprocal of both sides gives
[tex]R_1=28[/tex]Which is our answer!
