After resistor R1 and R2 are connected in parallel, they are connected to the power supply with voltage u, and the main circuit current is I. The current passing through R1 and R2 is I1 and I2 respectively. It is tried to prove that I1 = [R2 / (R1 + R2) Try to prove that I1 = [R2 / (R1 + R2)] I That is to say, I1 is equal to R2 divided by the sum of R1 and R2, and then multiplied by I}

After resistor R1 and R2 are connected in parallel, they are connected to the power supply with voltage u, and the main circuit current is I. The current passing through R1 and R2 is I1 and I2 respectively. It is tried to prove that I1 = [R2 / (R1 + R2) Try to prove that I1 = [R2 / (R1 + R2)] I That is to say, I1 is equal to R2 divided by the sum of R1 and R2, and then multiplied by I}

Because the resistance value of R1 and R2 in parallel:
R＝R1×R2/(R1＋R2)
Because:
R/R1＝[R2/(R1+R2)]
So:
R/R1＝I1/I
I1＝I×R/R1＝I×[R2/(R1+R2)]

Three resistors R1, R2 and R3 are connected in parallel in the same circuit, and their resistance values are 30 Ω, 20 Ω and 10 Ω respectively. The current passing through them is I1, I2 and I3 respectively. Then I1: I2: I3 is ()
A. 6：3：2B. 2：3：6C. 3：2：1D. 1：2：3

Because R1: R2: R3 = 30 Ω: 20 Ω: 10 Ω = 3:2:1, so I1: I2: I3 = 1r1: 1r2: 1r3 = 13:12:1 = 2:3:6