①R1和R3的串联,R1取最大值,所以R3=6/0.2-20= 10Ω
②R2和R3并联再和R1串联,IR3=6V/10=0.6A,PR3=6VX0.6A=3.6W,IR2=6V/30=0.2A,PR2=6VX0.2A=1.2W,PR2/PR3=1/3
①R1和R3的串联,R1取最大值,所以R3=6/0.2-20= 10Ω
②R2和R3并联再和R1串联,IR3=6V/10=0.6A,PR3=6VX0.6A=3.6W,IR2=6V/30=0.2A,PR2=6VX0.2A=1.2W,PR2/PR3=1/3