a) k hở => (R3//R2)
RTđ=\(\dfrac{U}{I}=\dfrac{60}{3}=20\Omega\) (I=Ia nhé )
Mặt khác ta có \(Rtđ=\dfrac{R2.R3}{R2+R3}=\dfrac{x.x}{x+x}=20=>x=40\Omega\)
Vậy R2=R3= 40 ôm
b) k đóng R1//R2//R3=>U1=U2=U3=U=60V
Ia1=I1=2A=>R1=\(\dfrac{U1}{I1}=30\Omega\)
Cách 1 \(\dfrac{1}{Rtđ}=\dfrac{1}{R1}+\dfrac{1}{R2}+\dfrac{1}{R3}=\dfrac{1}{30}+\dfrac{1}{40}+\dfrac{1}{40}=\dfrac{1}{12}\)=>Rtđ=12 ohm
Cách 2 I2=\(\dfrac{U2}{R2}=\dfrac{60}{40}=1,5A;I3=\dfrac{60}{40}=1,5A\)=>I=I1+I2+I3=2+1,5+1,5=5A=>Rtđ=\(\dfrac{U}{I}=\dfrac{60}{5}=12\Omega\)
Vậy.................