1/R=1/R1+1/R2
1/8=1/40+1/R2
解得:R2=10欧
1)电源电压:U=IR=0.75*8=6V
2)I1=(1/5)*0.75A=0.15A
3)I2=(4/5)*0.75A=0.6A
4)R2=10欧