请分析并解答，谢谢~~
热心网友
∵R1、R2、R3并联 == U=I1R1=I2R2=I3R3 ==I1:I2=R2:R1, I2:I3=R3:R2∵I2=1A ==I1=R2:R1(A), I3=R2:R3(A)∵R1=4Ω ==I1=U/R1=U/4∵P3=12W ==U×I3=12W ==I3=12/U∵I=4.5A ==I1+I2+I3=4.5 ==U/4+1+12/U=4.5 ==U=6V or U=8V当U=6V时I1=1.5A (由I1:I2=R2:R1) ==R2=6Ω根据P=U^2/R ==R3=3Ω当U=8V时I1=2A ==R2=8Ω根据P=U^2/R ==R3=16/3Ω(注：因为P=U×I,所以I3=P3/U，再根据I2:I3=R3:R2，可以算出R3，可用来做题目的验证)