Sđmđ: $(R_{2}//R_{3}//R_{4})ntR_{1}$
a, $R_{234}=\dfrac{1}{\dfrac{1}{R_{2}}+\dfrac{1}{R_{3}}+\dfrac{1}{R_{4}}}$
$=\dfrac{1}{\dfrac{1}{30}+\dfrac{1}{30}+\dfrac{1}{30}}=10$ ôm
$⇒R_{td}=R_{1}+R_{234}=15+10=25$ ôm
b, Vì $U_{2}=U_{3}=U_{4}$, mà $R_{2}=R_{3}=R_{4}$
$⇒I_{2}=I_{3}=I_{4}=0,5A$
$⇒I_{1}=3.I_{2}=3.0,5=1,5A$
c, $U_{MP}=I_{1}.R_{tđ}=1,5.25=37,5V$
$U_{1}=I_{1}.R_{1}=1,5.15=22,5V$
$⇒U_{2}=U_{3}=U_{4}=U_{MP}-U_{1}=37,5-22,5=15V$
