khi k mở,ampe kế A chỉ 0,5A.tính R2?
k mo mach gom
[(r1 nt r2)//(r3nt r4)] nt Ra nt r5
...
r2=x
r12=12+x
r34=4+12=16
r1234=16(x+12)/(x+12+16)
x+12=y
Rtd=R1234rar5=16y/(y+16)+1+15
=(16y+16(y+16))/(y+16)
Ia=Im=U/Rtd=12.(y+16)/[16(2y+16)]=0,5=1/2
<=>3(y+16)=4(y+8)
y=3.16-4.8=16(3-2)=16
R2=16-12=4(ohm)