Question

若R1与R2串联接入电路中电流为0.2A，并联接入同一电路中总电流为0.9A，已知R2=30欧，求R1和电源电压U

R2是30欧!!!!!=3=别弄错了!!!!式子弄出来了但是不会解!!!!

Answer 1

已知串联时
U=0.2*(R1+R2)=0.2(R1+30)=0.2R1+6
R1=(U-6)/0.2=5U-30
并联时
U=0.9(R1*R2/(R1+R2))=0.9(R1*30/(R1+30))=27R1/(R1+30)
将R1带入上面公式得:
U=27(5U-30)/(5U-30+30)
5U^2=135U-810
解方程得
U=9V或18V
R1=15欧(U=9V时)或
R1=60欧(U=18V时)

Answer 2

根据题意列出下式：
(R1 +30) * 0.2 = 1/(1/R1 + 1/30) * 0.9 = U
=> R1 = 15 且 U = 9 ；或者 R1 = 60 且 U = 18 .

Answer 3

有公式0.2*(R1+30)=U和公式U/R1+U/30=0.9，两式结合可得R1=15欧，U=9V或R1 = 60欧，U = 18 V.

Answer 4

慢慢解吧，R1=60欧，U=18V

追问

= =+不是有两种答案吗

追答

ok，或R1=15欧，U=9伏