Question

Answer 1

Ta có: \(R_{tđ}=\dfrac{R_1.R_2}{R_1+R_2}\)

\(\Rightarrow30=\dfrac{90R_1}{R_1+90}\)

\(\Rightarrow30R_1+2700=90R_1\Rightarrow60R_1=2700\)

\(\Rightarrow R_1=45\left(\Omega\right)\)

Answer 2

\(\dfrac{1}{R1}=\dfrac{1}{R}-\dfrac{1}{R2}=\dfrac{1}{30}-\dfrac{1}{90}=\dfrac{1}{45}\Rightarrow R1=45\Omega\)