4. R1//R2//R3
$\frac{1}{Rtd}$= $\frac{1}{R1}$+$\frac{1}{R2}$+$\frac{1}{R3}$
$\frac{1}{Rtd}$=$\frac{3}{9}$
Rtd=3Ω
7.
a.K mở
R3 nt ( R2// (R1 nt R4))
R14=R1+R4=10Ω
$\frac{1}{R14,2}$= $\frac{1}{R14}$+$\frac{1}{R2}$= $\frac{4}{15}$
=> R14,2= $\frac{15}{4}$
Rtd= R142+R3=$\frac{15}{4}$ +3=$\frac{27}{4}$ Ω
I= $\frac{U}{R}$ = 10: $\frac{27}{4}$= $\frac{40}{27}$
b.K đóng
(R1 nt R4)//R2//R3
R14=R1+R4=10Ω
$\frac{1}{Rtd}$= $\frac{1}{R14}$+$\frac{1}{R2}$+$\frac{1}{R3}$
Rtd= $\frac{5}{3}$ Ω
I= $\frac{U}{R}$ = 10: $\frac{5}{3}$ =6 A
