Bài 1:
$U_{1}=10V$
$R_{1}=10$ ôm
$R_{2}=15$ ôm
$R_{3}=25$ ôm
$R_{4}=40$ ôm
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Sđmđ: $[(R_{2}ntR_{3})//R_{4}]ntR_{1}$
$I=I_{1}=\dfrac{U_{1}}{R_{1}}=\dfrac{10}{10}=1A$
$⇒I_{2}+I_{4}=I_{1}=1$
$⇔\dfrac{U_{4}}{R_{23}}+\dfrac{U_{4}}{R_{4}}=1$
$⇔\dfrac{U_{4}}{15+25}+\dfrac{U_{4}}{40}=1$
$⇔U_{4}=20V$
$⇒U=U_{1}+U_{4}=10+20=30V$
$R_{23}=R_{2}+R_{3}=15+25=40$ ôm
$R_{423}=\dfrac{R_{23}.R_{4}}{R_{23}+R_{4}}=\dfrac{40.40}{40+40}=20$ ôm
$R=R_{423}+R_{1}=20+10=30$ ôm
Bài 2:
$U'=U_{5}=U=30V$
$I_{5}=\dfrac{U_{5}}{R_{5}}=\dfrac{30}{30}=1A$
$⇒I=I_{5}+I_{1}=1+1=2A$
