a, $K_{1}$ đóng, $K_{2}$ mở
Sđmđ: $[(R_{1}//R_{7})ntR_{2}]//(R_{4}ntR_{3})$
Ta có: $R_{34}=2R=2.30=60$ ôm
$R_{17}=\dfrac{R}{2}=\dfrac{30}{2}=15$ ôm
$R_{172}=R_{17}+R=15+30=45$ ôm
$R_{tđ}=\dfrac{R_{172}.R_{34}}{R_{172}+R_{34}}=\dfrac{45.60}{45+60}=\dfrac{180}{7}$ ôm
b, $K_{1}$ mở, $K_{2}$ đóng
Sđmđ: $(\{[R_{5}ntR_{6}nt(R_{4}//R_{7})]//R_{1}\}ntR_{3})//R_{2}$
$R_{47}=\dfrac{R}{2}=\dfrac{30}{2}=15$ ôm
$R_{5647}=2R+R_{47}=2.30+15=75$ ôm
$R_{15647}=\dfrac{R.R_{5647}}{R+R_{5647}}=\dfrac{30.75}{30+75}=\dfrac{150}{7}$ ôm
$R_{315647}=R+R_{15647}=30+\dfrac{150}{7}=\dfrac{360}{7}$ ôm
$R_{tđ}=\dfrac{R.R_{315647}}{R+R_{315647}}=\dfrac{30.\dfrac{360}{7}}{30+\dfrac{360}{7}}=\dfrac{360}{19}$ ôm
c, $K_{1};$ $K_{2}$ đóng
Sđmđ: $[(R_{1}//R_{7}//R_{4})ntR_{3}]//R_{2}$
$R_{147}=\dfrac{1}{\dfrac{3}{R}}=\dfrac{30}{3}=10$ ôm
$R_{1473}=R+R_{147}=30+10=40$ ôm
$R_{tđ}=\dfrac{R.R_{1473}}{R+R_{1473}}=\dfrac{30.40}{30+40}=\dfrac{120}{7}$ ôm
