(2) when t=0℃ and Rt=20,
According to ohm's law, I = ur = ur0+rt = 3v20ω+20ω =? 0.075A,
The voltage U0 = IR0 = 0.075A× 20Ω =1.5v on the constant resistance;
(3) When the temperature is high, the resistance value of the resistor is large, and the resistance value of the fixed-value resistor is too small relative to the resistor, resulting in obvious changes in the voltage representation.
So the answer is: (1) rt = 20+12t;
(2) When the temperature is 0℃, Xiao Ming observes that the reading of laboratory voltmeter is1.5v;
(3) When the temperature is high, the resistance value of the resistor is large, and the resistance value of the fixed-value resistor is too small relative to the resistor, resulting in obvious changes in the voltage representation.