\(R_{12}=R_1+R_2\)

                                            = 4 + 8

                                             = 12 ( Ω)

                                 Điện trở tương đương

                           \(R_{tđ}=\dfrac{R_{12}.R_3}{R_{12}+R_3}=\dfrac{12.8}{12+8}=4,8\left(\Omega\right)\)

                                   ⇒ Chọn câu : B 

 Chúc bạn học tốt

Vì R1 nt R2
=>R1,2 = R1 + R2 = 4 + 8 = 12 (Ω)
Vì R3 //  R1,2
=> Rtd = R_1,2 . R₃ / R_1,2 + R₃ = 4,8 (Ω)
Nếu đúng thì bạn hãy tick cho mình nha.
 

Mach nối tiếp hay song song vậy bạn?

\(R1ntR2\Rightarrow P2=U2.I2=I2^2R2=Im^2.R2\)

\(\Rightarrow P2=\left(\dfrac{Um}{Rtd}\right)^2.R2=\left(\dfrac{12}{R1+R2}\right)^2.R2\)

\(\Rightarrow P2=\dfrac{12^2.R2}{\left(R1+R2\right)^2}=\dfrac{144R2}{\left(6+R2\right)^2}=\dfrac{144}{\dfrac{\left(6+R2\right)^2}{\sqrt{R2}^2}}=\dfrac{144}{\left(\dfrac{6}{\sqrt{R2}}+\sqrt{R2}\right)^2}\)

\(\Rightarrow\left(\dfrac{6}{\sqrt{R2}}+\sqrt{R2}\right)^2\ge\left(2\sqrt{6}\right)^2\ge24\left(AM-GM\right)\)

\(\Rightarrow P2=\dfrac{144}{\left(\dfrac{6}{\sqrt{R2}}+\sqrt{R2}\right)^2}\le\dfrac{144}{24}\le6W\Rightarrow P2max=6W\)

\(dấu\) \("="\) \(xảy\) \(ra\Leftrightarrow\dfrac{6}{\sqrt{R2}}=\sqrt{R2}\Leftrightarrow R2=6\Omega\)

\(R_Đ=\dfrac{U^2_Đ}{P_Đ}=\dfrac{10^2}{10}=10\Omega;I_{Đđm}=\dfrac{P_Đ}{U_Đ}=\dfrac{10}{10}=1A\)

a)\(R_{12}=R_1+R_2=8+7=15\Omega\)

\(R_{tđ}=\dfrac{R_{12}\cdot R_Đ}{R_{12}+R_Đ}=\dfrac{15\cdot10}{15+10}=6\Omega\)

b)\(U_m=R_{tđ}\cdot I_m=6\cdot1,8=10,8V\)

\(I_1=I_2=I_{12}=\dfrac{U_{12}}{R_{12}}=\dfrac{U}{R_{12}}=\dfrac{10,8}{15}=0,72A\)

\(A_1=U_1I_1t=R_1I_1^2t=8\cdot0,72^2\cdot10\cdot60=2488,32J\)

c)Công suất: \(P=U\cdot I=R\cdot I^2\)

Mặt khác: \(R_1ntR_2\Rightarrow I_1=I_2=I\) mà \(R_1>R_2\)

Nên \(P_1>P_2\)

R2//(R1nt[R5//(R3ntR4))

\(=>R1345=R1+\dfrac{R5\left(R3+R4\right)}{R5+R3+R4}=7\Omega=>Rtd=\dfrac{R2.R1345}{R2+R1345}=14\Omega\)

\(=>I3=I4=I34=>U5=U34=I34.R34=0,5.\left(R3+R4\right)=3V=>I5=\dfrac{U5}{R5}=1A=>I1=I5+I34=1,5A=>U1345=U2=1,5.R1345=10,5V=U2=Um=>I2=\dfrac{U2}{R2}=1,5A\)

a.Rtd=R1+(R2*R3)/(R2+R3)

B,Rtd=((R1+R2)*R3)/(R1+R2+R3)

a) Rtđ=R1+((R2*R3)/(R2+R3))

b)Rtđ=((R1*R2)/(R1+R2))+R3

\(R1ntR2:R=R1+R2=8+10=18\left(\Omega\right)\)

\(R=\dfrac{R1.R2}{R1+R2}=\dfrac{8.10}{8+10}=\dfrac{40}{9}\left(\Omega\right)\)(R1//R2)

\(R_{td}=R_1+R_2\\ R_{td}=8+10=18\Omega\)