(1) When the slider moves to the left, the current representation becomes larger, indicating that the lower left terminal of the sliding rheostat must be connected to the circuit in series according to the principle of one up and one down, so the two terminals connected to the circuit are AC or AD;

(2) (1) According to the circuit diagram, when the voltmeter indicates 0, the current in the circuit is the largest, and the ammeter indicates that it is not 0, so the sixth set of data has errors;

② As can be seen from the table, the current I in the circuit is 0. 1A when U2=5V, and I=0.2A when U2=4V.

∫ The total voltage in the series circuit is equal to the sum of the sub-voltages, and the power supply voltage remains unchanged.

∴u=5v+0. 1a×r 1=4v+0.2a×r 1，

Solution: r1=10Ω, u = 6V.

③ The total resistance in the series circuit is equal to the sum of all resistances,

According to ohm's law, the current in the circuit:

I = ur 1+R2 = 6v 10ω+R2，

Electric power consumed by sliding rheostat:

P2 = i2r 2 =(6V 10ω+R2)2r 2 =(6V)2( 10ω)2r 2+20ω+R2，

∵ In mathematics, a+b≥2ab,

∴( 10ω)2r2+r2≥2( 10ω)2r2×r2=20ω，

That is, when (10Ω) 2R2 = R2, R2 =10Ω, the sliding rheostat consumes the largest electric power.

So the answer is: AC or AD; 6; 10; 10.