a) Ta có : ( R1//R2)ntR3
=> Rtd= (R1.R2/R1+R2) +R3
<=> Rtd = (8.12/8+12) +5 =9,8 ( ôm )
b) Ta có :P1 = I1².R1= 0,3².8=0,72 (w)
P2 = I2².R2 = (U2²/R2²).R2=(U2²/R2)=U12²/R2= U1²/R2=(I1.R1)²/R2=(0,3.8)²/12=0,48(W)
P3 = I3².R3= I12².R3=(I1+I2)².R3=(0,3+0,2)².5=1,25(W)
Mà P=P1+P2+P3=>P=0,72+0,48+1,25=2,45(W)