,a, sơ đồ
b, có: \((Rb1//Rđ)ntRb2\) .
đèn sáng đúng định mức \(=>\left\{{}\begin{matrix}U\left(đ\right)=U\left(đm\right)=3V=Ub1\\I\left(đ\right)=I\left(đm\right)=0,4A\end{matrix}\right.\)
\(=>R\left(đ\right)=\dfrac{U\left(đ\right)}{I\left(đ\right)}=\dfrac{3}{0,4}=7,5\left(om\right)\)
\(=>Ub2=Um-Ub1=12-3=9\left(om\right)\)
\(=>Ib1=\dfrac{Ub1}{Rb1}=\dfrac{3}{30}=0,1A\)
\(=>Im=I1+Ib1=0,4+0,1=0,5A=Ib2\)
\(=>Rb2=\dfrac{Ub2}{Ib2}=\dfrac{9}{0,5}=18\left(om\right)\)
\(=>Rtp=Rb1+Rb2=18+30=48\left(om\right)\)
Vậy....
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