Bài 1:
a. \(R=R1+R2=10+15=25\Omega\)
b. \(I=I1=I2=U:R=7,5:25=0,3A\left(R1ntR2\right)\)
\(\Rightarrow\left\{{}\begin{matrix}U1=I1.R1=0,3.10=3V\\U2=I2.R2=0,3.15=4,5V\end{matrix}\right.\)
c. \(R'=\dfrac{R3.R}{R3+R}=\dfrac{5.25}{5+25}=\dfrac{25}{6}\Omega\)
\(U=U3=U12=7,5V\) (R3//R12)
\(\left\{{}\begin{matrix}I=U:R=7,5:\dfrac{25}{6}=1,8A\\I3=U3:R3=7,5:5=1,5A\\I12=I1=I2=U12:R12=7,5:25=0,3A\left(R1ntR2\right)\end{matrix}\right.\)
Bài 2:
a. \(R=\dfrac{R1.R2}{R1+R2}=\dfrac{30.60}{30+60}=20\Omega\)
b. \(U=U1=U2=12V\)(R1//R2)
\(I=U:R=12:20=0,6A\)
\(\Rightarrow\left\{{}\begin{matrix}I1=U1:R1=12:30=0,4A\\I2=U2:R2=12:60=0,2A\end{matrix}\right.\)
c. \(U'=U3+U=4+12=16V\)
\(I12=I3=I'=0,6A\left(R3ntR12\right)\)
\(R'=U':I'=16:0,6=\dfrac{80}{3}\Omega\)
\(\Rightarrow R3=R'-R=\dfrac{80}{3}-20=\dfrac{20}{3}\Omega\)
