There is an ideal transformer, the number of turns of the original coil is N1, the number of turns of the two secondary coils are N2 and N3 respectively, the voltage of the original and secondary coils are U1, U2, U3 respectively, and the current is I1, I2, I3, the resistance of the load resistance of the two secondary coils is unknown. In the following conclusions, the correct one is a U1: U2 = N1: N2, B I1: I3 = N3: N1, I1 / I2 = N2 / N1, C n1i1 = n2i2 + n3i3, D i1u1 = i2u2 + i3u3

There is an ideal transformer, the number of turns of the original coil is N1, the number of turns of the two secondary coils are N2 and N3 respectively, the voltage of the original and secondary coils are U1, U2, U3 respectively, and the current is I1, I2, I3, the resistance of the load resistance of the two secondary coils is unknown. In the following conclusions, the correct one is a U1: U2 = N1: N2, B I1: I3 = N3: N1, I1 / I2 = N2 / N1, C n1i1 = n2i2 + n3i3, D i1u1 = i2u2 + i3u3

ACD
A U1: U2 = N1: N2 ("the voltage is proportional to the number of turns" is always true)
C n1i1 = n2i2 + n3i3 ("input power equals output power" is always true)
D i1u1 = i2u2 + i3u3 ("input power equals output power" is always true)

Push down of U1: U2 = N1: N2 in transformer
It is known that the alternating current of the primary coil changes with time, and the induced alternating current is produced in the secondary coil due to mutual inductance
At the same time, U1: U2 = N1: N2 because the flux change rate of primary coil and secondary coil is the same. That is to say, this is based on the same flux change rate. Therefore, the induced electromotive force of primary coil and secondary coil depends on the ratio of turns
However, isn't U1 still given an AC voltage? Why is this push down using induced voltage? Even if the induced voltage here is the same as the given voltage, according to Lenz's law, shouldn't their directions be opposite? Then will the induced electromotive force offset part of the AC voltage? I think that the above push down is wrong
So, how is the formula derived?
If the voltage in the original coil of an ideal transformer is 0 and the current is 0, why is there I1 / I2 = N2 / N1,

You are right. For an ideal transformer, according to Lenz's law, the induced voltage and the applied AC voltage have the same magnitude and opposite direction, which can be completely offset. It is also for this reason that there is no voltage ratio added to the primary side, and the coil resistance is equal to the primary side current

There are two resistors, R1 and R2, with resistance values of 20 and 30 ohm respectively. After being connected to a circuit in series, what is the total resistance? After being connected in parallel, what is the total resistance?

Series R = R1 + R2
R=20+30
R = 50 Ω
Parallel 1 / r = 1 / R1 + 1 / r2
R=R1*R2/（R1+R2）
R=600/50
R = 12 Ω